natbib Guide [pdf, CTAN]
Originally implemented only for the natural sciences (hence the document title), this package builds more functionality into your citations and is a good choice for using styles. "The natbib package is a reimplementation of the LATEX \cite command, to work with both author{year and numerical citations. It is compatible with the standard bibliographic style les, such as plain.bst, as well as with those for harvard, apalike, chicago, astron, authordate, and of course natbib."