Try it sitting, facing each other first to give her confidence; move her hips with your hands at first. Once she gets the hang of it, lay back. She should be riding you like Roy Rogers, holding on to the headboard before you know it. Be sure to help her along and don't go blowing your load too soon.
Oh, she should also learn to "squat" on top of you, so she can bob her ass up and down on your erect midget. Also, some chicks are just lazy in bed and not into too much other than cunnilingus followed by a missionary jack hammer session, in which case a weekend visit to Camp HamHock is your only solution. I don't charge for this service nor do I have a weight limit, however, you will have to try to prevent her from falling in love with me, stalking me, erecting shrines in my honor, ect. afterwards.
1 more tip, get a book on sex or on that East Indian Hindu shit about sex (I forget the name of it) and read it with her one night. If there are illustrations, try to find one with a dude who isn't hung like a horse, it will just have her thinking about me again.