Learning how to sit the trot properly without looking like a sack of potatoes flopping around on top of your horse is one of the most challenging aspects of English riding. The sitting trot, however, is the foundation for a centered, strong canter seat. It is also required on most basic dressage tests. With time, patience and the help of a friend or your instructor, you can learn how to sit the trot without bruising yourself or your horse.
Your horse's legs move in diagonal pairs at the trot, giving the gaits its strong ONE-TWO motion. When you sit the trot, the movement is absorbed not by your seat but by your abdominal muscles and your back. As the horse trots, you feel as if you are bounced up and down. By learning how to relax and center yourself over the saddle through engaging your core (trunk) muscles, you can learn how to relax and sink into the seat, using your seat as an effective aid. The result is that you flow with your horse's motions, not against them, at the sitting trot. A good sitting trot makes horse and rider look like they are one, because the rider moves with, not against, the horse's back.
Preparation Before embarking on these exercises, it helps to be able to post to the trot in both directions. Ideally, you can pick up your diagonals without peeking at the horse's shoulder. The more you learn how to find your diagonals by feeling rather than by sight, the easier it will be to sit to the trot.
Working with a trainer or friend while your horse is on a lunge line is ideal because you can focus on your position without worrying about what your horse is doing. If that's not possible, work in an arena or riding ring on a small circle during these exercises. It helps to have a pommel strap clipped between the D-rings on the front of an English saddle. A standing martingale or just the neck strap from a standing martingale (without affixing it to the bridle or girth) can also be used.
My assumption is that you're riding in either a general purpose English saddle or a typical jumping saddle.
Always warm your horse up before sitting the trot. Warm up for at least five minutes at the loose and collected walk. Trot for ten minutes in both directions at the rising trot. Do two-point exercises for several minutes in both directions before attempting the sitting trot. This ensures that your horse's back muscles are warmed up before attempting to add your weight in the sitting trot to his back.
Exercise One: Saddle Grasp After warming up your horse, begin a posting trot in a clockwise circle, picking up the correct diagonal. At the corner of the arena, sink deeply into the saddle. Grasp the pommel of the saddle, the martingale strap or saddle strap affixed to the D-rings. Use the pommel or the strap to pull your crotch and seat down into the saddle.
Try to feel the diagonal motion of the trot and tighten your abdominal muscles to hold your back upright and straight. If your hands bounce, wait until you can work on a lunge line and knot up the reins during this exercise so you don't bump your horse's mouth. You can do this exercise alone by using your dominant hand to hold the reins, and your non-dominant hand to grasp the pommel. If your horse drifts to the inside, hold the reins in your outside hand to gently guide him back to the rail.
Exercise 2: Riding without Stirrups You knew this was coming, didn't you? Drop your irons. There are two great exercises without stirrups to help you sit the trot effectively.
1. Pull up your leathers and cross your stirrups over the pommel. This prevents the irons from banging into your horse's sides during the trot. Ride the sitting trot without stirrups, and then rise into a posting trot without stirrups.
2. Ride the long wall of the arena with stirrups at the posting trot. Then on the short walls, drop your irons and sit the trot. Pick up your irons again on the long wall and repeat. This exercise is a lot harder than it sounds. If you find yourself fishing about with the toe of your boot for your iron, you may need to pull into the center of the arena and cross your stirrups until you're confident riding without them. Alternatively, test your stirrup length; they may be too short, making it difficult for you to find your irons once you drop them.
An effective sitting trot moves with your horse, not against him. Some horses are bouncier than others, but even the bounciest trot can be accommodated in the sitting trot. Remember to always warm your horse up before working at the sitting trot, and if you feel like you are out of control, stop and return to gaits you're comfortable with before proceeding. Talk to your instructor if you have any questions.