Apart from the fitness and awareness of your position (in my view) it is important to always work on your posture on a horse every day that you sit on him. In England I trained with John F. Lassetter. I met him at an international competition. He caught my eye during the warm up because of the impeccable way he sat on his horse. His incredibly quiet and friendly hands and legs were especially enviable. I wanted that too. He is an amiable man, so we had a chat and he invited me to visit him at Goodwood. I did and I never left. John had gotten his extremely classic education as the first foreigner at the Spanish Riding School in Vienna, followed by a period at the Cadre Noir in Saumur. With him I just went back to the lunge line for sitting lessons, even though I was already an advanced rider, because that’s how they did the first year at the Spanish Riding School. Even after that it was repeated once a week, regardless of your level. I never succeeded to get such fantastic hand position as John, but I am still working on it and I am incredibly grateful for all that he has taught me.
A correct posture looks good. You sit upright, but without tension. You try to be light for your horse, in balance, with quiet hands and legs, without squeezing or interfering. It is important to follow the movements of your horse. That sounds simple and when you see famous riders it seems simple, even with such big moving horses. But how do you do it? Several things are needed for this. They are stamina, a supple back and loose hips. You should be able to release the tension in your body without losing your posture. When you say “sit up straight” to someone, muscles often get tightened. The trick is not to do that and do not to pull your head between your shoulders. Stay upright, lift your ribcage and keep feeling what happens underneath you. Picture your spine as a stack of loose cubes. Make sure that the blocks do not fall over. Imagine your head is attached to the ceiling with an elastic cord and above all: do not clamp with your legs.
Tension blocks movement
That is all nice and well, but sitting without tension or clamping is hardly possible if a horse tightens his back. Tension blocks movement. When a horse tightens his back he is uncomfortable to sit on. If you hold the reins too tight or pull them all the time or if you bump on his back or squeeze your legs too much, he strains his back. But if he does you can’t sit well. Here we have the chicken and egg question. What came first? Lesser gods blame the horse and reach for a draw rein. However, I don’t think it matters who starts. You’re the smarter of the two or at least you should be and you are the one who wants to ride him. So, it is up to you to break this negative spiral.
Try to not blame the horse but instead work on yourself. Punishing or forcing him does not relax his back. That requires training. Start with yourself by taking those sitting lessons and find a good instructor to work with. Correct posture is a matter of hard work.
I am not in favor of riding without stirrups a lot. The result is usually that riders clamp more to stay in balance. The correct riding position is with stirrups, so practice it that way. You can take them off for some gymnastics, but too long without is counterproductive and is not so nice for the back of your horse.
As long as you are not able to sit relaxed and your horse keeps his back tight, there is no point in attempting sitting trot. Stay in rising trot and try to take some sitting lessons, preferably on an experienced horse. Ride as many different horses as possible, which will help you develop your seat. Practice your posture and work on making your horse supple. Try occasionally (preferably on a circle) to consciously stay a little closer to the saddle while doing rising trot or go sitting for a quarter of a circle, after which you go back to rising again. That way it gets easier automatically.
Some horses are more comfortable than others. The more supple their back, the easier they are to sit on. “Dutch Design” is a big strong giant, who can easily carry me. Trot is his best gait, so sitting on him is like being in a comfortable chair. “Socrates” is a very fast elastic spring and he is very bouncy. Still, I can always sit on him just fine, because he relaxes his back well. However, I alternate with rising trot a lot to maintain that relaxation.
Working on posture problems is quite difficult. What you are used to becomes a habit. Changing that takes a lot of time and tenacity. Start figuring out what made you do it that way. I increasingly notice that riders take over the responsibility of their horse. What do I mean by that? I mean riders will do with their body what they actually want their horse to do in response to their help. In other words: the aid is not answered or it is ignored or incorrectly translated by the horse. You see riders sticking their elbow out on horses that are bent and leaning against the rider’s leg with their bellies on that side. You see raised shoulders or moving arms on horses that do not respond to the request for more impulsion. You see riders hopping up and down on horses that don’t bounce off the ground in piaffe. You see me flying out of the saddle when I try to make the canter changes bigger. So if you have something to correct in your posture, first check whether your aids are coming through.