tons of no stirrup work to really get the feel of your butt in the saddle ! let loose of your hips and let the horse carry you
I would also recommend just trotting without stirrups to open your hips and make your leg longer and as you're doing it focus on trying to wrap your legs around horses belly this will also help u to put your leg on more effectively. Once you will finish this exercise and you'll tale your stirrups back you might feel they r too short. Go a hole longer and then try some center, you should fell a difference and feel how your seat aid improved.
^^good advice! Also when you do use your stirrups sink into your heel and wrap your leg down and around the horse, drive with your butt but not too hard or over exaggerated, play around with it until you feel comfortable
Canter a lot with no stirrups and do some circle work too. Lunge work at the canter with your hands behind your head worked for me.