It is possible to write your own: the standard bibliography styles are distributed in a form with many comments, and there is a description of the language in the BibTeX distribution (see BibTeX documentation). However, it must be admitted that the language in which BibTeX styles are written is pretty obscure, and one would not recommend anyone who’s not a confident programmer to write their own, though minor changes to an existing style may be within the grasp of many.
If your style isn’t too ‘far out’, you can probably avoid programming
it by using the facilities of the custom-bib bundle. The bundle
contains a file makebst.tex, which runs you through a text menu
to produce a file of instructions, which you can then use to generate your
own bst file. This technique doesn’t offer entirely new styles
of document, but the custom-bib’s “master BibTeX
styles” already offer significantly more than the BibTeX standard set.
An alternative, which is increasingly often recommended, to use biblatex. Biblatex offers many hooks for adjusting the format of the output of your ‘basic’ BibTeX style, and a collection of ‘contributed’ styles have also started to appear. Note.bowever There are not as many of biblatex’s contributed styles as there are for BibTeX, and there is no custom-biblatex, both of which suggest that beginners’ röle models are hard to come by. As a result, beginners should probably resist the temptation to write their own contributed biblatex style.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=custbib