Developing a good seat. . . help!

Get someone to lunge you riding your horse, put a bucking strap on, then drop your stirrups (cross them over the saddle is safer) and canter without stirrups. Anytime you feel you're getting out of balance grab the bucking strap and pull yourself deep into the saddle.

Make sure you sit straight up in the saddle, your legs drape around the horses barrel (not pinching or that will pop you out of the saddle) and you've got your shoulders back and are looking straight ahead (not at ground, not left or right).

Make sure you're back on your seat bones. Do you have an instructor that can give you lunge line lessons? No stirrups!! It's torture, but nothing works better...

Exactly Anabel!

Did you know that those who are brought into the Spanish Riding School in Vienna, are not allowed to ride on their own for a whole year?

They are put on a lunge line, with no reins for a whole year - why? Because this forces them to ride SEAT first, establish their seats and teaches them to focus primarily on the more important aspects of their body, than the least important *unlike here in North America where 8 out of 10 riders ride hands first*

Do exatly this - not for a whole year, but have someone put you on a lunge like who is experienced to aid you - and drop your reins and leathers. You're seat will quickly say "HELLO! Remember me?!?!"

One of the big things I see riders doing today is trying to be still. You can't be still you have to move with the horse.

I rode with a lot of crazies early on in my dressage career. My first dressage trainer couldn't really teach anything beyond correct seat and position so I was basically stuck on the lunge. My second dressage trainer decided I was "unbalanced" (having never ridden a horse with big gaits) and put me on the lunge line on an imported Hanoverian state stud mare (read: VERY large gaits) and then had me touch my opposite foot with my hand in all gaits.
Then I had a jumping trainer who would remove her students stirrups for weeks and she would set up a chute, tie your reins up and give you a blindfold, the "crazy" part was that the jumps were at least 3' tall and the final oxer was always reminiscent of a free jumping oxer for a stallion testing. When I trained with her all I had to say about x-country was thank God there was no way to set up a chute out in the middle of nowhere!!

The good thing I got from all the crazies is that now a horse can absolutely bronc on me and I do not move. The last time I fell off a horse was dismounting and my horse spooked when my leg was already over and the time before that was getting wiped off on a wall by a mare that most of the trainers in my area won't touch with a 10' pole.
Lunge lessons in excess = you do not fall off.