http://www.zotero.org/support/requesting_styles
start with American Psychological Association (APA)

not necessarily, no - although you're certainly encouraged to try yourself, if you're requesting a style, you're supposed to follow the instructions in the link - i.e. steps 1), 2) and 3) there.
From what you've posted, APA looks like your best bet for step 1 - plus it'd make sense that a business/marketing professor largely follows a psychology style.

Ok so, I was able to figure out some parts:

<?xml version="1.0" encoding="UTF-8"?>
<style xmlns="http://purl.org/net/xbiblio/csl" xml:lang="de" class="in-text" >
<info>
<title>American Sociological Association</title>
<id>http://www.zotero.org/styles/asa</id>;
<link href="http://www.zotero.org/styles/asa"/>
<author>
<name>Julian Onions</name>
<email>julian.onions@gmail.com</email>
</author>
<category term="author-date"/>
<category term="sociology"/>
<updated>2008-10-20T16:32:11+00:00</updated>
<summary>The ASA style.</summary>
<link href="http://www.asanet.org/galleries/default-file/asaguidelinesnew.pdf" rel="documentation"/>
</info>
<macro name="editor">
<names variable="editor">
<label form="verb" text-case="lowercase" suffix=" "/>
<name and="text" delimiter=", "/>
</names>
</macro>
<macro name="series-editor">
<names variable="original-author">
<label form="short" text-case="capitalize-first" suffix=". "/>
<name and="text" delimiter=", "/>
</names>
</macro>
<macro name="anon">
<text term="anonymous" form="short" text-case="capitalize-first"/>
</macro>
<macro name="author">
<names variable="author">
<name name-as-sort-order="all" sort-separator=", " delimiter=", "
delimiter-precedes-last="always"/>
<label form="short" prefix=", " suffix="." text-case="lowercase"/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", "/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="access">
<group>
<text variable="URL"/>
<group prefix=" (" suffix=")">
<text term="accessed" text-case="capitalize-first" suffix=" "/>
<date variable="accessed">
<date-part name="month" suffix=" "/>
<date-part name="day" suffix=", "/>
<date-part name="year"/>
</date>
</group>
</group>
</macro>
<macro name="title">
<choose>
<if type="thesis">
<text variable="title"/>
</if>
<else-if type="book">
<text variable="title" font-style=""/>
</else-if>
<else>
<text variable="title" quotes="false"/>
</else>
</choose>
</macro>
<macro name="publisher">
<group delimiter=" " >
<text variable="publisher-place"/>
<text variable="publisher"/>
</group>
</macro>

<macro name="year-date">
<choose>
<if variable="issued">
<date variable="issued">
<date-part name="year"/>
</date>
</if>
<else>
<text term="no date" form="short"/>
</else>
</choose>
</macro>
<macro name="day-month">
<date variable="issued">
<date-part name="month"/>
<date-part name="day" prefix=" "/>
</date>
</macro>
<macro name="pages">
<label variable="page" form="short" suffix=". " text-case="capitalize-first"/>
<text variable="page"/>
</macro>
<macro name="edition">
<choose>
<if is-numeric="edition">
<group delimiter=" ">
<number variable="edition" form="ordinal"/>
<text term="edition" form="short" suffix="."/>
</group>
</if>
<else>
<text variable="edition" suffix="."/>
</else>
</choose>
</macro>
<citation>
<option name="et-al-min" value="4"/>
<option name="et-al-use-first" value="1"/>
<option name="et-al-subsequent-min" value="4"/>
<option name="et-al-subsequent-use-first" value="1"/>
<option name="disambiguate-add-year-suffix" value="true"/>
<option name="disambiguate-add-names" value="false"/>
<option name="disambiguate-add-givenname" value="false"/>
<option name="collapse" value="year"/>
<layout prefix="(" suffix=")" delimiter=", ">
<group delimiter=":">
<group delimiter=" ">
<text macro="author-short"/>
<text macro="year-date"/>
</group>
<text variable="locator"/>
</group>
</layout>
</citation>
<bibliography>
<option name="hanging-indent" value="false"/>
<option name="et-al-min" value="6"/>
<option name="et-al-use-first" value="1"/>
<sort>
<key macro="author"/>
<key variable="title"/>
</sort>
<layout suffix="">
<group delimiter=" ">
<text macro="author" suffix=" ("/>
<text macro="year-date" suffix="),"/>
</group>
<choose>
<if type="article-newspaper article-magazine" match="any">
<group delimiter=" ">
<text macro="title" prefix=" " suffix=","/>
</group>
<group prefix=" " delimiter=", ">
<text variable="container-title" font-style=""/>
<text macro="day-month"/>
<text variable="edition"/>
<text variable="page"/>
</group>
</if>
<else-if type="thesis">
<text macro="title" prefix=" " suffix="" quotes="false"/>
<group prefix=" " delimiter=", ">
<text macro="edition" />
<text macro="editor" suffix="."/>
<text variable="genre"/>
<text macro="publisher"/>
</group>
</else-if>
<else-if type="book">
<group delimiter=" ">
<text macro="title" prefix=" " suffix="."/>
<text macro="edition" />
<text macro="editor" suffix="."/>
<text macro="publisher"/>
</group>
</else-if>
<else-if type="chapter">
<group delimiter=" ">
<text macro="title" prefix=" " suffix="."/>
<group class="container" delimiter=", " suffix=".">
<group delimiter=" ">
<text macro="pages"/>
<text term="in" text-case="lowercase"/>
<text variable="container-title" font-style=""/>
</group>
<text variable="volume" prefix="vol. "/>
<text variable="collection-title" font-style=""/>
<text macro="editor"/>
</group>
<text macro="publisher"/>
</group>
</else-if>
<else>
<group suffix="," >
<text macro="title" prefix=" "/>
<text macro="editor" prefix=" "/>
</group>
<group class="container" prefix=" " suffix="." delimiter=", ">
<text variable="container-title" font-style=""/>
<group delimiter=" (">
<text variable="volume" />
<text variable="page"/>
</group>
</group>
</else>
</choose>
<text prefix=" " macro="access" suffix="."/>
</layout>
</bibliography>
</style>

resulting in:
(Slater and Narver 1998)

Slater, Stanley F., Narver, John C. (1998), Customer-Led and Market-Oriented: Let's Not Confuse the Two, Strategic Management Journal, 19 (1001-1006.

What is still missing? What was I not able to do?
- After the volume in the Bibliography I figured out the bracket, but after it I would like to have the month and then a closing bracket.
- My main problem: single and multi citations: Instead of (Slater and Narver 1998) I need (Slater/Narver 1998).
-Did I change the et. al. rule correctly? Also can someone please explain what the following lines stand for?

<option name="et-al-min" value="4"/>
<option name="et-al-use-first" value="1"/>
<option name="et-al-subsequent-min" value="4"/>
<option name="et-al-subsequent-use-first" value="1"/>

Hope to have a response soon :-) Cheers

PS: I only implemented the changes for the journals...which parts must I change for a book?

That already helped a lot. Thanks a million.

I installed the style and have a problem when referring to specific pages:

(Kohli/Jaworski 1990:4-6)

However, it should look like this:

(Kohli/Jaworski 1990, S. 4ff.)

- no colon...instead a comma followed by a space and 'S.' which stands for the German word 'Seite' meaning page.
- if referring to only one page: add nothing
- if referring to page page 4 and the following one: add f.
- if referring to pages 4 to x: add ff.

I already changed my headline to
<style xmlns="http://purl.org/net/xbiblio/csl" xml:lang="de" class="in-text" default-locale="de-DE" >

So theoretically, 'p.' should be changed to 'S.' anyways...when adding the right variables.

However, I don't know where this part is within the style. The whole style does not contain a written ':', so I have no idea where it comes from.

I already searched for German stuff in the forum, but in this one http://forums.zotero.org/discussion/9186/ebd-ibid-in-footnotes/

the link to http://www.mediafire.com/?sharekey=c14691802f6af3bac2b435915e8821d7e04e75f6e8ebb871 doesn't work anymore.

This might be a common problem, so I hope it doesn't take 2 long for you...really appreciate it.

Think I'm all good then..that's the last major thing, which I so far can't figure out by myself.

Cheers!!!

I actually figured out the colon part, but I was confused since the style update didn't work in word and hence I was referring to "There's no colon in the style." :) ...works now.

I solved the page part via group delimiter..ok true..quite a workaround, but should do for now.:

<option name="collapse" value="year"/>
<layout prefix="(" suffix=")" delimiter=", ">
<group delimiter=", S. ">
<group delimiter=" ">
<text macro="author-short"/>

Various English citation styles use p. for single page and Pp. for multiple. Isn't there a way to use that rule to get what I need?

- if referring to only one page: add nothing
- if referring to page page 4 and the following one: add f.
- if referring to pages 4 to x: add ff.

(Kohli/Jaworski 1990, S. 6) = perfect
(Kohli/Jaworski 1990, S. 4-5) should be (Kohli/Jaworski 1990, S. 4f.)
(Kohli/Jaworski 1990, S. 4-6) should be (Kohli/Jaworski 1990, S. 4ff.)

By 'can't be done automatically' you mean I would have to change the citations by hand in my document? Changing suffix fields in the *.csl would change the outcome for all page citations, unless I could somehow catch the amount of pages to work on a if-else-clause?!

What do you think? If this really can't be done, guess it's something for the next zotero release. :)

Crush, note that you have to insert the range manually in the word processor plugin anyway — so on your end, you just have to insert "4ff." instead of "4-6" in the 'page' field while inserting a citation.

@mark: lol, of course you're right! 4ff. it is :-)
@adamsmith: thanks for your contribution.
@soltera: how random :) another one who doesn't want to use refworks or endnote :)

if you post it somewhere accessible I'd also be happy to upload it to the repository.

this is an in-text citation style - there are no footnotes/endnotes that are controlled by Zotero, thus you can't chose between them.
For general footnotes/endnotes use the controls in your word processor of choice.

as I said - it depends on the style. Only so called note styles that work with footnotes natively have that option enabled -
There are three types of styles in Zotero
1. Notes - e.g. Chicago Manual of Style - all citations are in footnotes or endnotes, usually (but not necessarilly) in extended form
2. in-text - citations are part of the written text. Most commonly in the form (Author Year, 19xx)
3. Numeric - citations are numbered consecutively and usually (but not necessarily) listed in the order of appearance in the bibliography

Of these, only the first one gives you the footnote-endnote option, because these are the only styles that create footnotes/endnotes.
which style do you want to use? (and yes, there most likely is a way to add a footnoted style, depending on what exactly you want that may be more or less work, though)

try the International Organization style in the repository - that will be very close

Hey, sorry...totally forgot about this one.

Adam, here's the style I ended up using for my paper:

<?xml version="1.0" encoding="UTF-8"?>
<style xmlns="http://purl.org/net/xbiblio/csl" xml:lang="de" class="in-text" default-locale="de" export.bibliographyLocale="de">
<info>
<title>Homburg</title>
<id>http://www.zotero.org/styles/Homburg</id>;
<link href="http://www.zotero.org/styles/asa"/>
<author>
<name>Julian Onions</name>
<email>julian.onions@gmail.com</email>
</author>
<contributor>
<name>Thomas W</name>
<email></email>
</contributor>
<category term="author-date"/>
<category term="business"/>
<updated>2008-10-20T16:32:11+00:00</updated>
<summary>Prof Homburg, Uni Mannheim</summary>
</info>
<macro name="editor">
<names variable="editor">
<label form="verb" text-case="lowercase" suffix=" "/>
<name and="text" delimiter=", "/>
</names>
</macro>
<macro name="series-editor">
<names variable="original-author">
<label form="short" text-case="capitalize-first" suffix=". "/>
<name and="text" delimiter=", "/>
</names>
</macro>
<macro name="anon">
<text term="anonymous" form="short" text-case="capitalize-first"/>
</macro>
<macro name="author">
<names variable="author">
<name name-as-sort-order="all" sort-separator=", " delimiter=", "
delimiter-precedes-last="always"/>
<label form="short" prefix=", " suffix="." text-case="lowercase"/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="author-short">
<names variable="author">
<name form="short" delimiter="/" delimiter-precedes-last="always"/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="access">
<group>
<text variable="URL"/>
<group prefix=" (" suffix=")">
<text term="accessed" text-case="capitalize-first" suffix=" "/>
<date variable="accessed">
<date-part name="month" suffix=" "/>
<date-part name="day" suffix=", "/>
<date-part name="year"/>
</date>
</group>
</group>
</macro>
<macro name="title">
<choose>
<if type="thesis">
<text variable="title"/>
</if>
<else-if type="book">
<text variable="title" font-style=""/>
</else-if>
<else>
<text variable="title" quotes="false"/>
</else>
</choose>
</macro>
<macro name="publisher">
<group delimiter=" " >
<text variable="publisher-place"/>
<text variable="publisher"/>
</group>
</macro>

<macro name="year-date">
<choose>
<if variable="issued">
<date variable="issued">
<date-part name="year"/>
</date>
</if>
<else>
<text term="no date" form="short"/>
</else>
</choose>
</macro>
<macro name="day-month">
<date variable="issued">
<date-part name="month"/>
<date-part name="day" prefix=" "/>
</date>
</macro>
<macro name="pages">
<label variable="page" form="short" suffix=". " text-case="capitalize-first"/>
<text variable="page"/>
</macro>
<macro name="edition">
<choose>
<if is-numeric="edition">
<group delimiter=" ">
<number variable="edition" form="ordinal"/>
<text term="edition" form="short" suffix=","/>
</group>
</if>
<else>
<text variable="edition" suffix="."/>
</else>
</choose>
</macro>
<citation>
<option name="et-al-min" value="4"/>
<option name="et-al-use-first" value="1"/>
<option name="disambiguate-add-year-suffix" value="true"/>
<option name="disambiguate-add-names" value="false"/>
<option name="disambiguate-add-givenname" value="false"/>
<option name="collapse" value="year"/>
<layout prefix="(" suffix=")" delimiter=", ">
<group delimiter=", S. ">

<group delimiter=" ">
<text macro="author-short"/>
<text macro="year-date"/>
</group>
<text variable="locator"/>
</group>
</layout>
</citation>
<bibliography>
<option name="hanging-indent" value="false"/>
<option name="et-al-min" value="6"/>
<option name="et-al-use-first" value="1"/>
<sort>
<key macro="author"/>
<key macro="year-date"/>
<key variable="title"/>
</sort>
<layout suffix="">
<group delimiter=" ">
<text macro="author" suffix=" ("/>
<text macro="year-date" suffix="),"/>
</group>
<choose>
<if type="article-newspaper article-magazine" match="any">
<group delimiter=" ">
<text macro="title" prefix=" " suffix=","/>
</group>
<group prefix=" " delimiter=", ">
<text variable="container-title" font-style=""/>
<text macro="day-month"/>
<text variable="edition"/>
<text variable="page"/>
</group>
</if>
<else-if type="thesis">
<text macro="title" prefix=" " suffix="" quotes="false"/>
<group prefix=" " delimiter=", ">
<text macro="edition" />
<text macro="editor" suffix="."/>
<text variable="genre"/>
<text macro="publisher"/>
</group>
</else-if>
<else-if type="book">
<group delimiter=" ">
<text macro="title" prefix=" " suffix=","/>
<text macro="edition" />
<text macro="editor" suffix="."/>
<text macro="publisher" />
</group>
</else-if>
<else-if type="chapter">
<group delimiter=" ">
<text macro="title" prefix=" " suffix="."/>
<group class="container" delimiter=", " suffix=".">
<group delimiter=" ">
<text macro="pages"/>
<text term="in" text-case="lowercase"/>
<text variable="container-title" font-style=""/>
</group>
<text variable="volume" prefix="vol. "/>
<text variable="collection-title" font-style=""/>
<text macro="editor"/>
</group>
<text macro="publisher"/>
</group>
</else-if>
<else>
<group suffix="," >
<text macro="title" prefix=" "/>
<text macro="editor" prefix=" "/>
</group>
<group class="container" prefix=" " suffix="." delimiter=", ">
<text variable="container-title" font-style=""/>
<group delimiter="">
<text variable="volume" />
<text macro="day-month" prefix=" (" suffix="), "/>
<text variable="page"/>
</group>
</group>
</else>
</choose>
<text prefix=" " macro="access" suffix="."/>
</layout>
</bibliography>
</style>

crush?

The current implementation of the reference style for the chair of Prof. Homburg, University of Mannheim can be found here: https://github.com/UB-Mannheim/UniMA-styles/blob/master/universitat-mannheim-lehrstuhl-prof-homburg.csl