Speed Up Sites with htaccess Caching
produke writes "Increase your page load times and save bandwidth with easy and really effective methods using apache htaccess directives. mod_headers to set expires, and max-age, and cache-control headers on certain filetypes. The second method employs mod_expires to do the same thing -- together with FileETag, makes for some very fast page loads!"
Its in the comments on that site, but remember, you're always better off putting this kind of stuff in your httpd.conf as opposed to .htaccess files. htaccess files reduce performance on your webserver.