Use your seat, sink into their back, and sit straight
Seat! Slow your movements and your horse should follow. If you rise slowly horse will slow down
Teach him voice commands.
Voice commands if trained
Sit deep, take a deep breath and stop the movement of your hips in whatever gait your in. Depending on how responsive your horse is depends on how he will respond. He may pick it up right away or you may need to use a little rein just to clarify what you want. Also using a verbal cue, when you ask for the downward
Sit very very deep and say whoah!
Put your chin up in the air and tighten your abs while leaning back. My trainer had me on a lunge and told me to do this and my horse slowed down. (She had my reins on a tie down)
Use your seat and thighs!
Breathing helps a lot! It depends how your horse is trained. I've seen some that stop when you sit deep and lean back and others that stop when you lean forward a bit. Test with your horse to see which they respond to. Then use this to do downward transitions and with some practise it will work.
Sit deep in the saddle and keep your leg very still
Use your seat. It's really easy for me because my mare is very sensitive. All I have to do is lean back a bit in the saddle and she stops.
My trainer has us do breathing exercises. Taking deep breathes in and increasing you energy makes your horse move forward, and exhaling/ decreasing your energy and sitting back in the saddle which signals to the horse you want to stop.