I wanted to be able to fold long BibTeX files to display a condensed list of the references so I could quickly scan the contents of the file. To do this, I created a function in my .vimrc which folds .bib files on the @ symbol which prepends each reference entry in the file. This is the excerpt from my .vimrc:
" Set folding function for bibtex entries
function! BibTeXFold()
if getline(v:lnum) =~ '^@.*$'
return ">1"
endif
return "="
endfunction
au BufEnter *.bib setlocal foldexpr=BibTeXFold()
au BufEnter *.bib setlocal foldmethod=expr
The final two lines which start with au BufEnter set the method of folding to expression and identifies the expression to use (BibTeXFold()).