imma drop some knowledge on you bro! Been is this situation a couple times actually
1. Keep doin what your doing if she wants to she will if not you guys will be considered "close friends"
2. Even if you dont score with her now, when shes outta the relationship or looking for fun she will think of you.
3. Do nice stuff for her but not obvious like flowers or some shit. Just take her out to coffee or something.
4. Do fun and exiting activities that her boyfriend would not do so she thinks your the more exiting and spontaneous person which will make her want to do more crazy stuff. (like you)
5. Dont EVER EVER EVER EVER EVER EVER tell her your feelings for her.....if you are cool bout it she will know. dont go to great lengths and if it happens it happens but it probably will eventually.
Good Luck man hope this helps....
Hugo