One of my (and my g/f) favorites is to start out missonary. To get into it, I bring my knees up as far forward as possible so that I am now kneeling verticle (or perpendicular to her). I then take her by the back of knees and push them against her. At that point, bring her legs together and bring them back to you against your upper body. Then grab her strongly by the thighs. Lean back a little and and lift. The obtuse angle will help you hit the g-spot and it's not a terribly tiring posision so you can keep it going that way for a while.
It kinda ends up like being a reverse doggie style. Works quite well
