Generally a bouncing lower leg is due to a gripping knee/thigh. This puts you in a precarious position - if your horse bucks you're already over the dash and it won't take much to put you on the ground.
I have found so far, that the most effective method of combating this is to practice standing in your stirrups. You physically cannot grip with your knee and thigh, and if your heels come up you'll wobble all over the place - thus dropping them down quick smart!
Start with riding in short stirrups, up in 2 point with your fists just in front of the horse's wither. You should be able to stay in this position, at walk, trot and canter. It hurts like buggery if you're not used to it, but I assure you that your position and stability in the saddle will improve out of sight.
When you have developed the muscle to hold this position, start dropping your stirrups down a few holes at a time, and bring your upper body upright. This will engage a whole new set of muscles and once again, will hurt like buggery and you'll probably keep tipping forward. It won't take long for your to learn exactly where to place your lower leg, to prevent you from tipping forward or backwards. Another bonus is that you'll be working your core - essential if you want to be a dressage rider.
Again, you should be able to hold this position, with a perfect ear-shoulder-hip-heel line, in walk, trot and canter.
By the time you have dropped your stirrups back to normal, and can get your backside into the saddle, you'll probably want to lower your stirrups a couple of holes again. Your lower leg will be so much more stable, you'll be clued into the importance of using your core to balance, not your legs, and you won't be gripping with your thigh and knee.