As challenging as posting the trot may be, many riders find sitting the trot even more difficult. Here are some tips to help you learn to maintain a secure seat at the trot.
Sitting the trot is a bit like trying to pat your head and rub your tummy at the same time. You have to relax your leg muscles so that your grip is not pushing your seat out of the saddle while using your abdominal and back muscles to absorb the motion and to follow it. Bareback and no stirrup lessons will help you get the sitting trot faster.
Lunge Line Training
It helps to start on a lunge line so you don't have to worry about your horse and can concentrate on your position. Remove the stirrups and leathers as crossing them over the pommel usually leaves an uncomfortable lump under your thigh. Removing the stirrups helps you deepen the seat and improve your balance. Alternatively, try lengthening your stirrups a hole or two to help relax your legs.
You want to be sitting as deeply as possible, so do some stretching exercises before you mount—particularly a sideways lunge, where you start with feet apart and slowly transfer most of your weight to one foot, bending the knee and stretching with the other leg straight. You should feel your inner thigh muscles protest a bit. After enough repetitions, you should also feel as if your pelvis has spread a bit wider.
Once mounting, place your palms on the pommel and push yourself up out of the saddle, at the same spreading your legs in a wide "V." Lower your seat back into the saddle with your legs still stretched and feel the difference in your seat.
Next grasp the pommel (or a strap running through the D rings if that's more comfortable) and pull yourself even further down and forward into the saddle. Now you're ready to sit the trot.
At first aim for only a few strides before repositioning yourself. Gradually add more strides, but always stop with the first bounce and regroup. This will be easier on both you and the horse. Stop as soon as you feel yourself starting to bounce. Once you begin to bump with every stride your horse's back will stiffen in defense and the bouncing just gets worse.
At first, you just need to completely relax, like a wet noodle, and learn to follow the horse's motion. Relax your back, seat, legs, shoulders, and arms. If you start to bounce, relax, breathe, let your muscles relax and follow. Resist the temptation to tighten up. Be conscious of when you are tightening your knees and thighs and don't turn your toe out.
Instructors recommend leaning back as far as you can comfortably, keeping your shoulders open. Dangle your legs until you find your seat and rhythm. Sit back up once you get it.
Think of reverse posting. Drive your seat down into the horse on every beat instead of rising up. Try to really feel your horse's stride and push down and forward into the saddle with each stride. This will help you get the feel of the rhythm.
If you can borrow a horse that works through his back at the trot, so that the motion is more longitudinal than up and down, you'll find your "Eureka" moment will come much earlier and more easily.