The phone doesn't keep much of a cache, though it does maintain a history of the pages it accesses. To prevent a page from being kept in this history, set its expiry date to "already expired".
The closest thing I can think of to clearing the cache is to exit the service. Press the services button and that will clear the application page history. An Init:AppStatus might do the same thing, but Key:Services is probably more straightforward.
I'm not aware of any way to view the phone's current history.
Reference:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guide_chapter09186a008040575b.html
