How to clear the cache from memory

Linux has a supposable good memory management feature that will use up any "extra" RAM you have to cache stuff. This section of the memory being used is SUPPOSED to be freely available to be taken over by any other process that actually needs it, but unfortunately my linux (two distros now, mandriva i586, and mandriva x86_64) thinks that cache memory is too important to move over for anything else that actually needs it...