In my experience, forward motion is key to avoiding a rear.
If the horse does achieve the rear you can kick him forward, however that usually results in a forward leap into the rear and then forward motion, occasionally ending in a buck.
Once the horse begins to rear, you can jab a knuckle into his spine, just above the withers, it's uncomfortable and will lower the rear to about 2 feet, but will not stop or fix it.
You can also grab your rein and yank the horse to the left or right while he is on the upswing. This will knock him off balance and force his feet to the ground. Once his feet hit the ground you need to push him forward. Something to watch for in a smart horse is the counter action, which would be for him to pull his head in the opposite direction that you plan to pull. You'll already be off balance and he'll end up yanking you right out of the saddle.
Another option is the take a short stick and smack him directly between the ears while he is on the UP swing. He has to be going up when you smack him, otherwise it doesn't really have the effect you are looking for, which is to make him think he's bumping his head on something... (this is also where the crack an egg and rubber hose method come in)
The above are all things I've tried in the past with my chronic rearer. For me, they are good counter-actions, but I've never found an actual "cure" for him.
I was once told by an old-timer to tie a piece of bailing twine from his halter to his tail. I don't think I did it right because when he reared, all I managed to do was rip out a chunk of tail.
I've been told to dismount and flip the horse onto his back during a rear but have never, and will never resort to that method.
Is there a reason for the question? Curious.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"