die.net - Optimizing Page Load Time:

"Instead of relying on the browser to revalidate its cache, if you change an object, change its URL. One simple way to do this for static objects if you have staged pushes is to have the push process create a new directory named by the build number, and teach your site to always reference objects out of the current build's base URL. (Instead of <img src="http://example.com/logo.gif"> you'd use <img src="http://example.com/build/1234/logo.gif">. When you do another build next week, all references change to <img src="http://example.com/build/1235/logo.gif">.) This also nicely solves problems with browsers sometimes caching things longer than they should -- since the URL changed, they think it is a completely different object."