Do this at your own risk.

Using a registry editor like Resco.
Go to My Device\Windows\profiles\guest\temporary internet files\content.IE*

with in this file there are temp internet files usually that are named g43kd99 or something crazy like that. open them and delete all the files. They are usually save web page pictures. But i cleared out 20mg of storage. Make sure its in a temporary internet file or it will screw up you pda. Then you can use resco's search engine and find everything bigger than what ever size you want and delete them that way too.

You can do this easier from Pocket IE by launching IE, going to Menu->Tools->Options, and selecting the Memory tab. From there, you can see the Delete Files button which will also clear those files for you.