Ok, so here's the deal.  My girlfriend and I often find ourselves lying on my bed doing things.  Whatever they might be.  It could be watching movies, watching TV, just talking, cuddling, playing video games whatever.  Sometimes we are making out.  Recently, I've become more... adventurous; and I've been rubbing her vagina (through her pants, through her underwear, or directly on it. All of 'em).  I have asked her about it, and she says she likes it.  And she does... invite me to do it sometimes.  But I feel like sometimes I'll do something that she REALLY likes, and I find it difficult to get the same thing again.  I asked her for tips, to show me what she likes.  But she kind of giggled it off and said "you're so cute when you're confused ^_^".  Well... it's good that I'm cute and all, but I would really like to be able to do better than I am.  So, basically I'm asking for tips and tricks.  From guys, things you know your girl likes.  From girls, things you know you like.

EDIT: I forgot to mention, she doesn't want me inserting my fingers in her vagina, only rubbing.