The gallop is generally performed out hacking or perhaps if are able to ride in a field, rather than in a school. This is because it requires a good amount of space to get up speed and decelerate again.
The gallop is not something that involves being "out of control" as this pace can be varied and controlled just like any other. Gallop is most often ridden in a forward seat that is very similar to jumping position. To achieve this, as when jumping, you may wish to shorten your stirrups. The aim is to hover just above the saddle supporting yourself on your knees and stirrups without pulling on the reins to hold your position.
Some horses may get exciteable and strong in the gallop and in this case it can help to bridge your reins, as this gives you a secure contact. Bridging your reins means once the rein has passed between your thumb and fore finger it goes across the horse’s neck to your other hand where it is held between your thumb and forefinger. You can do this with one or both reins. Keep your hands low, resting them on the horse’s neck if you wish.
In order to gallop, first go into a canter and then adopt a forward seat; then use both legs to ask the horse to gradually accelerate. When you want to stop steady the pace with your reins and sit back down into the saddle. Here are some further tips.
Don’t try to gallop if you’re control in canter is uncertain!
Pick a good piece of level or slightly uphill ground which is straight and gives you plenty of time to see ahead and stop if need be.
If you’re not sure you’ll be able to stop practice in an enclosed field at first.
Don’t try and go too fast. You should feel that you could stop at any moment should you need to.
Remember that the riding arena is too small a space in which to practice galloping!
Oh and enjoy it too!
If you should find that you can’t stop then keep calm, sit down in the saddle and sit up straight. If the pace is too fast or unbalanced for you to sit to reasonably then stay in forward seat. Don't get into a tug-of-war with the horse by continually pulling on the reins but try short pulls on the reins, releasing in between until the horse listens. If this has little or no effect, and there is room the easiest thing to do is to ride ever decreasing circles. As the horse circles it will slow up to balance itself and then you can stop. If circling is not possible then place one hand firmly on the neck with the crest of the neck between your thumb and fingers and pull firmly on the other rein with a long pull and brief release until you have the horse again under control. If you ride sensibly you will very rarely, if ever, be really out of control.
Taken From Equine World