I used the "push and turn" method with a little help from its partner the "beat the sh1t out of it till it turns" method. Both in tandem worked wonders!

http://s.sears.com/is/image/Sears/00...sm=0.9,0.5,0,0

Works wonders on oil filters, I'd imagine it would make short work of a stuck oil cap.

These plastic oil caps are no joke man.
I took mine off while I was working on my car today just to see if I could lol..