I actualy almost landed a 720 by accident today. The most I've ever spun is 540. Anyway, I was hitting about a 20 footer. Even though most people say to spin with your hips, I was actually beginning the rotation in my shoulders. By the time I would leave the lip, my shoulders were already at 90. Once in ther air, just tuck and keep your head looking ahead of you to continue the spin... if you feel like you're going to overrotate, as the guy above me said, untuck and it should slow your spin down. Work your way up to it, warm up with some 540s and when you land, continue the spin so that you end up going forwards. It will get you in the mindset so when you land you're 720, you'll be ready.
Above all, best of luck!