Now for a fun little game you're going to play with her. Have you go down on you again until you are close. You are going to inform her that she can get of while she rides you. (this is an evil game, hehe) don't let her start until she gets you right to the edge with a blowjob. Have her hurry up and ride you.
Tell her to make it count.
Let her have fun while you get closer and closer...tell her to tell you when sea starting to get close...
And right before she really gets to the edge and gets off...make sure YOU cum. Help her off you.
Shes probably going to be very frustrated wih you...so make sure she is nice and restrained. Tie her ankles together or apart or whatever you would like...