Originally Posted by EnglishGuy
Sink your weight into your heels and wrap your legs around, gripping with your calves and thighs.
Sorry, EnglishGuy, but I'm going to have to disagree with this little bit of your advice! You don't want to grip with any of your leg. When you grip, you're tensing more than is required to sit a gait. In truth, your lower leg should really be loose. Not so loose that it's flailing, but you really don't need it to sit at all. What you DO need is your thigh, but not to grip with, just as a base for all your body's weight to sink into which will, in turn, "anchor" you to your horse's back.
I'm a part of another forum where a similar question was asked and here was my response:
Sitting a gait smoothly emanates from your pelvis and lower back. Being centered over this area of your body will help you out TONS, as far as your balance is concerned. The first thing to look at is where your lower back is at when you're riding. It should be the same at the halt, walk, trot and canter in the three-point (full seat) position and this is very centered over your pelvis. Ask your trainer to tell you when you look completely straight in the saddle, then memorize the sensation, so you can recognize when you have it and when you don't.
Now, once you're sitting up straight, you want to find the base of your balance: your seat bones. While you're sitting up nice and straight, take your hands and place them directly underneath your seatbones. You should be able to feel their weight and an almost sharp feeling to them on your hands. Be sure to keep in mind that these are what you are balancing on! Equate that to balancing a bowling ball on a couple of pencils: not the most balanced situation! What you should do, while you still have your hands under your seatbones, is roll your pelvis forward and underneath, so the bottom of your pelvis is moving forward, and your lower back is pushing backward. The new sensation on your hands will be less acute pressure from your seatbones and more "meat" from your buttocks. Go back to the analogy now. Instead of bowling ball balanced on two pencils, you now have a bit more cushion, like a bowling ball resting on a couple of firm pillows. This cushion, along with providing more shock absorption, creates a greater surface area on which to balance. If you keep this position while you're cantering (or trotting), it will make feeling and moving with the motions much easier.
Another important part in balancing at any gait is settling the weight of your body across your inner thighs as well as your seatbones and buttocks. Be sure not to tense up any of your thigh muscles, and relax and recenter your seat bones before the transition to the canter (or trot). Think about all of your weight pouring from the top of your head right down into your thighs. By creating this weight in your thighs, you'll have lowered your center of balance even more, making staying with the motion even easier.
Also remember that one of the main shock absorbers when riding at gait is your lower back. Don't be afraid to let it act almost as a joint as your hips sink, rise, and swing in different directions with the motion of the horse. It's normal for it to be very mobile, in order to keep the top half of you from moving too much and upsetting your balance.
And one more thing...don't forget to breath!! Very imperative to a nice, relaxing ride. When you hold your breath, you often tense up muscles without even realizing it and the whole idea behind keeping your balance is to be able to relax into the motion, making it comfortable for you and more importantly the horse!!