There's so many things you can try! Just remember, no matter what you try, to have patience. It's most likely a phase. My son still even does it every once in a while and it's because he just doesn't understand how to deal with the feelings. When he went through the initial phase we tried:
- Telling him no, over and over
- Putting him in his crib and walking away
- "Thunking" his hand and telling him no (a light flick with your finger)
- Holding his arm behind his back (or both arms if your kid is anything like mine)
- Removing from current situation and distracting with another (just be careful it's not a reward of any sort)
In the end, I think it was just the consistency of telling him no EVERY time and creating a negative consequence to be associated with the behavior.
My son also did and still does throw himself back. If we are somewhere carpeted, we let him go. Hitting his head a couple times has certainly taught him. Otherwise, we hold his arm/hand just enough to soften the blow and let him. I've even "thunked" the back of his head a time or two (throwing the head into the chest is NOT pleasant). Again, it's the consistency of creating a negative consequence to the behavior consistently. If you let them get away with it even once, the cycle starts over. They will start to understand hitting and throwing themselves back does not get them what they want and they'll move on.
IT WILL PASS!!!