I had a friend at a warm up jump in the Grand Prix warm up at Spruce and McLain Ward was schooling Adrienne Sternlicht telling her to look up at trees to improve her balance. We could all do better I think 
I’m really tall and used to pinch with my knees. Pilates really helped with that. There are so many free instructors on YouTube, I love Move with Nicole personally.
Gymnastics and bounces with wide hands were super helpful for finding my balance without using the neck The bounces should also be helpful for your horse. I would also practice doing them one handed. This can help build a little muscle memory.
Having a trainer remind you right before take off to stretch away with your upper body is a huge help. You really should just be closing your hip angle and following the mouth. Sometimes on takeoff, I will remind myself to keep my leg down and in front and that corrects a lot of the rest of the my position. Other trainers say hip back, which can also be a helpful visual.
If you pinch with your knees, be careful with no stirrups. In your quest to get more fit, you may start pinching even more when you get tired. Either have the discipline to stop and take a breather, or work to make sure you’re squeezing at the calf. I don’t post without stirrups, but cantering and walking without have been super helpful. Also riding with way too long of a stirrup can be great practice and better for your horses back as you get stronger.
The last thing I would say is to make sure your flexibility isn’t also hindering you. If the backs of your legs and hips are tight, it can impair your ability to stay in the center of the horse with your hip closed but your chest open. Forward stretches are great for that.
