From my experience (or lack thereof) it seems that girls like it when you put your tongue all in there, and lick. Doing so gets the most reactions.
Also, do not forget the clitoris, lick that some too..it's reputed to feel really good
Addendum:
http://www.askmen.com/love/love_tip/36_love_tip.html
Check out the whole sexuality section...good stuff.