<META HTTP-EQUIV="Expires" CONTENT="Tue, 01 Jan 1980 1:00:00 GMT">
<META HTTP-EQUIV="Pragma" CONTENT="no-cache">
Above META tags should be placed in between your <HEAD> and </HEAD> tags. Also note that the specified date should be a date in the past so that the browser will immediately discard the cached copy or not cache it at all.
Why use two tags you ask? Although "Expires" tag is properly handled by most browsers, some older browsers may still look for the "no-cache" tag. If you're not worried about users with old browsers, using the "Expires" tag alone is sufficient and will make it possible to specify the exact time to expire content.