I agree with Faye about true bolters, thankfully, I don't think there are very many true bolters out there.
I also agree with Boldstart. Bridge your reins, hold your hands along side your horse's neck by his mane and give squeezes on the reins every few seconds, like squeezing out a sponge. If the horse has really set his jaw against you alternate your squeezes left then right rather than at the same time, steer your horse as best you can by looking to were you want to go, by looking to where you want to go your body should give your horse the right signals as well, so stay focused to were you want to go, be determined.
Think also that this is a resisting strong elastic feeling on the reins and don't lock your elbows, the horse will be pulling against himself, do not pull the reins back or back and up at all. You will only upset your own balance by doing so. Also do not lean forward or back, stay centred over your horse and think of becoming very bottom heavy, letting all of your weight sink straight down to the ground. Also, be very aware of your breathing, stay calm and breathe deeply and calmly and be aware of not tensing up anywhere in your body. Yes, this seems like a lot to think about and maybe sounds a bit weird, but it really works, the more aware you are of your own body and how to control your own body the better you will be able to control your horse and get him back under control.
I also would never suggest bailing off, unless your life really depends on it, like I think someone mentioned, going over a cliff as an example.
Also, spend lots of time in the saddle working on developing a good seat and secure position. Also, it is a good idea to work on your fitness out of the saddle, especially core strength. You will be able to stay in control better if something really were to happen.