I also use my seat to slow my guy. He is trained that if I inhale deeply and raise my hand slightly, he will lope. Sit deep and hum, and he slows. It helped me to work on the "speed control" bareback at the walk, so that he learned the feel of my shifting weight. When I was cooling him out I used to play with this, and now he does it well.
Absolutely serpentines, etc help immensely at walk and trot. We used to put out cones and do cloverleaf patterns, do serpentine lines, whatever to break the monotony.
Good luck-remember, it takes MUCH longer to retrain a horse than to train it right in the first place. You are retraining. That takes tons of time and consistency. My guy was in full time training (ridden 5 days a week by a trainer) for 6 month to finally "get it".