All the action for have a secure seat go through the heels! If your heels are too low it contracts all the muscles of the leg and therefore it is more difficult to remain fixed while following the movement and being relaxed.
Unfortunately there is no exercise done for the seat, because each person does not have the same plate. Someone who has never been riding can have a superb plate, even better than that of a rider who climbs for 10 years ...
But there are a few tricks, the first is training, so you have to ride a horse. The second is that it is better to climb with stirrups a little too long when we an unsecure seat, because it forces us to well placed heels. The third one is logical, you need a saddle or you are well installed in it. The fourth is that it is necessary to think to push well with the bottom of the abs and not with the buttocks and the thighs. To succeed you can imagine that you have weight in the heels but you have to keep your back and your legs relaxed.
I hope this helps :)