Really? At a full gallop I'd be looking for a water landing. Most of the times I have come off a horse I really didn't have time to prepare for the "take-off." In all seriousness, I am not trying to be a smart-a#$, but how do you fall properly? Between the point of separation from the horse, being suspended in mid-air like a Warner Brothers cartoon character, and the eventual splat on the ground, there is not a lot of time to get in position.
Falling properly involves a bunch of techniques, that you need to practice to get fixed in your muscle memory.
1) Don't tense up your body or the fall will be a nasty one and you increase your risk of a broken bone or three. Be as floppy as possible.
2) ROLL. Especially if you're flying head first. The difference between rolling and not rolling could be the difference between a bit of a sore body and a lifetime in a wheelchair. By flopping, you make it easier for your body to roll and to avoid your head and spine (esp neck) taking the brunt of the impact.
3) Do NOT stick out your hands. A hard one as it goes against all instinct. But putting out your hands to stop a fall will prevent the floppiness for starters, and also dramatically increases the chances of breaking wrists or arms, or dislocating a shoulder.
4) As soon as you're on the ground, ROLL again. Get away from those hooves. Be aware of your situation.
Best way to learn good falling techniques is to watch race falls. Not only do those jockeys have to avoid hurting themselves in the fall itself, they also have to get away from the many pairs of hooves thundering around them. This stuff takes lots of practice to get right, they actually have training courses for it (for jockeys mostly). But you can practice at home by falling or being pushed onto cushions til you learn to relax your body.
I can tell you that, having been bucked off countless times in the past few years, thanks to my regular practice of good falling techniques the worst injuries I have received from falls have been a broken toe (coming out of the stirrup) and mild concussion (my helmet came off when I landed and I hit a stone). I have a very weak neck from a gymnastics accident aged 7, I cannot sleep with my head turned left without waking up unable to move it. Yet with coming off in a head-first manner so many times I have never once experienced neck pain, because I relax and roll.
As for emergency dismounts - yes these can be done even at full gallop, although that generally reduces the chances of you remaining on your feet. Kick both feet out of the stirrups (a good idea anyway to prevent dragging), make sure your hands aren't caught up in the reins or mane, and swing a leg. Generally you'll end up landing on your feet then on your butt, but try and be as relaxed as possible in all your muscles, and roll away from the horse and any horses behind you. Why learn an emergency dismount? Surely it's better to stick on? Well, not if your horse is going toward a cliff or barb-wire fence, or is galloping blind down a hill or into traffic!
OP - look up the one rein stop. This has to be taught in a safe environment first so the horse knows to flex when you grab one rein. Failing that, try and aim a galloping horse at a hill or even a small incline, if there is any around. Generally, for some reason, even a very fit (as in competing endurance) horse will gallop up the hill (and it is much easier to keep your seat going up a hill) then stop at the top, or at least break into a trot, in which case you can gracefully slip over the side.