Working on your balance will also help your seat. Practice lots of frog position (put your stirrups up as high as they can go and ride around, like a jockey). Walk, trot, canter, change directions, transition up and down and practice your halts.
Once you can do ALL that, start seeing how long you can stay up while trotting, then see how long you can stay up while cantering.
Learn how to post the canter (go up one stride, down one stride. It's awkward but once you figure it out it's fun)
Post at a different beat. Instead of the up-down 2 beat post, go up for 2 beats, down for 1. Try different combinations of this - 3/2, 3/1 etc.
These things are also great to do for your warmup.