This rider (Ashleigh Rauen on Greta) says she is working to get her seat deeper but, in trying to do so, she loses the weight in her heels. The depth of the seat is only a product of the other aids. You can’t make it deeper by controlling it; a deep seat is created when the leg is loose, allowing the seat to sink down. If you think about pushing the seat down, you will tighten it, which causes it to come farther away from the saddle.
My advice is to ride without stirrups; think of keeping a loose knee and letting your whole leg relax. Don’t try to keep the heels down because that means keeping your toes up, which takes muscle. We don’t want any muscle. If you have a totally relaxed leg, then, when you take your stirrups back, they will feel shorter. Your heels will be down because your stirrups prevent your toes from coming down where gravity takes your whole leg. Any time you feel your heels coming up, take your knee away from the saddle. This will also make your seat deeper. Heel up and light seat go together; both can be remedied by taking the knee away.
This photo shows a good example of a broken line (in what should be a straight line) from bit to elbow. The elbows also appear a little tight in an effort to keep the hands and the horse’s head up. I think that taking the knee away and allowing the seat to come down will encourage the horse to be more down behind and up in front. This will eliminate the need to keep the hands high to keep the horse’s head up.
This rider has a very good upper-body position—head up and shoulders back—and she does this, importantly, in a relaxed way. The one flaw is that there’s a slight arch in her lower back. I think this, too, will begin to be rectified by allowing her knee to loosen and her seat to deepen. Think of sitting on a couch at home. Even if you slouch, immerse yourself in that immense relaxation.
I know it’s hard to tell from the single moment of a photo, but it looks like this rider tries a little too hard. She has a good position from a technical standpoint, but I want to tell her: It’s time to let yourself go and allow yourself to feel. Riding is like dancing; once you learn the steps, you let feeling create the action within the steps.