In order to give proper advice, I think it would help to know your ages.
One thing that's always welcomed by girls is romance. Hold her hand in public (and mean it), buy her flowers (or other little gifts) now and then, write her letters and leave them in her locker or on her car's windshield, take her out to dinner and/or a movie without expecting anything sexual afterwards, kiss her withOUT quickly moving on to touching. When I was still a teen (I'm female), I remember being curious about sex/"making out" yet deep-down NOT liking the way guys would always try to see how "far they could go." Sometimes you just want a guy to be sweet to you without putting his hands all over your body. I know you were asking for sex advice rather than romance advice, but I assume you're in high school and honestly, the advice I'm giving you here is the truth for 99% of teen girls, even if they don't admit it when you ask them. (A lot of females are afraid to turn their "boyfriends" away from them by saying they want to slow things down and just have romance without sex.) So another question I have for you is, do you truly love your girlfriend, or do you just like her and want to have fun with her?