The right position for dressage is not something that has been invented at random.
Riding horses started a few hundred years before Christ. Before that we just ate them. Since then man has developed the position in such a way it’s easy to maintain for a human being, while at the same time signals can be given to the horse that make sense to him. The way to sit on a horse properly hasn’t changed for ages. And it’s important. Why? Try running around with a child on your neck. If the kid is waving its arms or leaning to one side, you’ll have a lot of trouble keeping your balance and stay on a straight line. Same goes for the horse.
Practise
As it is the easiest way, why is it so hard to maintain that position or to stay in it in the first place? Everything you want to be good at, you’ll have to practice, to develop muscles at the right places. In the old days, horses were means of transportation or used for war. Long days in the saddle. Nowadays we rush in after work for an hour at max, while sitting in offices or classrooms for most of the day. Not the same. But it’s worth to work on it. How can you ask your horse to execute difficult body movements when you are on his back like a sack of potatoes? How can you give your aids so light and specified, if you don’t have control over your limbs? Are you sure your hands are not moving involuntary while you do something with your legs?
Fitness training
Good riders always work on their position. Sitting lightly but upright, balanced, hands and legs quiet without squeezing. It’s not a bad idea to have special lessons for seat and posture once a week. Specialized fitness training can help to overcome certain problems with position. Flex chair sessions or rein tension devices can be of use, to make you more aware of what you do with your body.