I agree, relaxing is key but sometimes that is easier said than done. I have found that a visual in your mind helps---thinking if it as pushing a swing (particularly at the canter) or your back and hips as a spring (especially for the trot) have always been good visuals for me. But sometime even that isn't enough and you need some exercises! Taking the reins in one hand and holding the pommel with the other hand is quite helpful, especially if you can have a friend or trainer lunge you so that you aren't having to worry about steering. An exercise that I think might be even more helpful is to work without stirrups. Without stirrups it's pretty hard to bounde so it really lets you develop your seat. Meanwhile, it works wonders for your legs and overall will strengthen you tremendously as a rider. If you are taking lessons, ask your trainer to help you work on this. If you don't have a trainer, it might be easiest to ask a friend to lunge you so that you can again, focus on you and not your horse. You could even consider going without stirrups and holding the pommel for a little extra help when you're first giving it a go. If no one can lounge you, it would still be fine to just do this on your own. A few important tips 1) Do this on a day that your horse is being well behaved---it isn't fun trying to handle a frisky horse while learning no stirrups! 2) Cross your stirrups over your horse's withers (just in front of the pommel) so that they aren't bouncing around and scaring your horse, 3) Be very aware of your legs---push yourself to maintain proper leg position---toes up and elongate your legs---long and low. Most people have a tendency to pinch with their knees and to stiffen up as they start feeling unbalanced. Remember, relax, maintain even pressure throughout the length of your leg (not just your knee), and lower that leg---it may almost feel as though your leg is nearly straight, but keep those toes up! 4) Do whatever gaits and manuevers you are comfortable doing but push yourself in terms of how long you go to enhance strength. If you only feel comfortable trotting at first, only trot and work towards building up to cantering. You can also work up to posting, two-point, jumping, etc without stirrups---anything is possible as you get stronger and all the while, your seat will be benefiting.