The best way to accomplish this is to find some documentation somewhere for whatever web browser(s) you are developing for an send the appropriate headers. Each browser has it's own way of specifically denying cachable information. It's a pain in the ass.

Actually, the way that I'm leaning towards right now is to cache the information locally and send the cached information to the client. This works best under mod_perl, because you don't have to worry about the overhead of starting up a perl processor to handle the return of cached data (like you would normally).

I've found that IE tends to cache based on what it's user settings are and tends to ignore the Expires: header. And even with the CGI module, you can return whatever headers (status and otherwise) you want.