I recently started using FreeOTFE.
http://www.freeotfe.org/mobile_site/index.html
Its free and pretty easy to use. You decide how much encrypted storage you want and create an encrypted volume file that size. You can then mount the volume and it shows up in the file explorer as another storage card. Anything written to this virtual storage card is encryped on the fly. Unmount the volume file and no one can see what is written to it. All they'll see is a single large encrypted file.