Um.. haha make the horse buck more so you can get more practice? lol!
Just hang onto the saddle and lean back- if you go forward you're going to eat dirt. Grip with your lower legs.
If your horse keeps bucking though, you don't want to know just hwo to sit it, you want to know how to put an end to it because bucking is NOT on, get him/her checked out by a chrio before you ride again, if she hasn't bucked much before she might be out somewhere.
As to putting and end to it, well, yep keeping her head up is good, but if you can't do it, then rip her head around with one rein and spin her off your inside leg, then ask her nicely to go out again without going stupid on you.