After you two have been fooling around for a while let her get some action. Step up to the plate and finger her. Girls always return the favor. As long as you do a good job and get her off she will want to do the same for you. Notice I said get her off not have her fake it. When she is giving you a hand job say it isn't working. Most girls will then go down on you. Just make sure you tell her before your bust if you want her to do it again. Although some girls don't mind, especially if she is new at that she will appreciate it, maybe the next time you can get her to swallow.
Cheers
-Andrew