How do you sit to a canter on a horse?

First do a sitting trot, then squeeze the horse into a canter. Make sure to keep your heels down, shoulders back and push into the saddle.

Keep your heels down and relax into the saddle. Let your hips go with the movement of the horse (don't worry too much about bouncing at first). Don't drop your contact as you go into canter, and don't rest your hands on the horse's neck. Keep your elbows in and pretend you are carrying a tray. Keep your hands still so you don't jab your horse in the mouth. Keep your shoulders back and don't use the reins to balance yourself. Squeeze gently with your legs to keep the horse in canter. Make sure you're not still rising to the trot as you are asking for canter. Don't grip with your knees. Have some lunge lessons to help perfect your seat, the better your balance, the easier you will find it to sit to the canter. It sounds loads, so concentrate on a bit each lesson. I promise you it will come!

Shorten up your reins, and make sure you have a good forward trot. Sit to the trot and put your inside leg on the girth and outside leg just behind the girth and squeeze with your legs. If your horse/pony is well trained he/she should canter.

All of the above seems pretty accurate. To avoid bouncing around when you're in the canter, relaxation is key. Allow your arms to follow the movement of the horses neck. Relax your lower body. Allow your seat to follow the rocking motion of the canter. One helpful trick is to pretend you're polishing the saddle with your seat, but don't try to force this motion, just allow yourself to follow the horse's movement. Also, you probably already no this, but you can canter a horse from a walk. Sometimes this is easier than a sitting trot, because you don't bounce around as much. At a walk, do a half halt and ask for the canter. The idea of the half halt is to allow your horse to get collected and ready for the transition. The transition is easiest either when approaching the corner in a ring or riding in a circle.