I ride bareback, not only does it help with your balance but also helps develop your seat. Also do lots of stirrupless trotting and cantering and make sure you sit back and get your bum into the saddle. My mom always said "when you're cantering I want those heels to be down and I want to see you cleaning that saddle with your butt" meaning she didn't want to see any bouncing!
Half seat at trot and canter pushing your heals down as far as you can then drop your stirrups and do the same thing over. Then go to sitting both without and with stirrups. Practice that way several times a week and you will see a difference in your leg position and seat ability.
One of the best exercises on deepening your seat and having a long leg is riding with no stirrups. While cantering and trotting you can hold onto the pommel of the saddle and push yourself deeper.