(From TBEventer2002, edited) As I am a hunter/jumper, I am very adament about having a stable lower leg. Since you are dressage (and I know two types of dressage people: those that rely soley on their seat and could care less how high their heels are or where their lower legs are and those that work on getting that wonderful long stable leg), I will have to break you into the world of jumping for just a minute.
LOTS and LOTS of no stirrups work and two-point jumping position. You can do all that in your dressage saddle, don't worry -- in fact, since it is harder in a dressage saddle, you should get very good at it!
For jumping position, concentrate on dropping the weight of your body through your heels -- don't jam them down! It might be easier for you to raise your stirrups a couple of holes at first so that you can raise your rear out of the saddle. Don't be afraid to balance on your horse's crest so that you aren't hanging onto their mouth. Try to grip with your thighs, but don't pinch with your knee (if you pinch, you'll lose your lower leg away from your horse's body and the exercise will be pointless). Do that at all three gaits.
Do lots of no stirrups work as well. Once you are good at two-point and no stirrups, try doing both at the same time (VERY hard, even for me!).
A little while at those exercises and you will have the tightest, prettiest legs out there! Good luck, and have fun!