Give pocket cleaner plus a whirl
http://www.freewareppc.com/utilities...anerplus.shtml
It clears out temp files, internet temp files, cookies etc
You could also try pocket mechanic. Clearing out unused registry entries screwed a few thing up on me though so be a bit more careful.
If you use AvantGo and have a lot of channels that can also take up a bit of room. Use something like RegKing to move them onto a storage card.