How to turn on my girlfriend?

Me and my girlfriend are a little young. 18 and 17. We don't have sex, and wont in a while but, sometimes, we engage into some sexy cuddling, where we sometimes touch each other and make out a lot. In these moments I feel very turned on, but she barely does, or I even think she doesn't. What can I do to turn her on, that doesn't involve penetration or any sexual activity either than just touching and kissing?.. Thanks :)