I use my rein position to tell my horse to increase and decrease speed at a gait as well as my seat. It is easiest to teach if you have a round pen of at least 60' with good footing so you can feel safe on a loose rein.
Let's start with the trot and I will use my cue system. On a loose rein, ask the horse to trot. Change your seat to two point position and push your hand as far as you can up his neck without letting your face get over or in front of the pommel. Cue with voice and or leg to get your horse to go as fast as he can without changing gait. Example: I use both legs at the girth for walk, trot, and slightly behind to speed up at these gaits.
Let the horse stay at the increased speed until you feel he is thinking about slowing down. At this point sit down and bring your hand back. If the horse slows, praise him. If he does not slow then rise up, push your hand forward and ask him to speed up again. If the horse breaks gait, turn his head toward the rail, change direction to the outside and pick up the trot again. Use the rail to help turn the horse as you want a definite stop going this direction and start going that direction to get the horse to break gait down. You also want to only pick up enough rein to get the change of direction and then give it all back.
There may not be a big change at first, but as your horse tires and starts to understand, the change will be more and more pronounced. You should work on the lesson until you have a strong working trot and as slow a trot as possible. The greater spread of speed the better the control you have. You may need to work on it several days depending on your horse. Look for improvements by at least 20mins of work.
Now when you go on trail give the horse as much rein as you can, I give it all. If the horse breaks gait, turn him back as you did in the pen and ask for the original gait. If he breaks going the new direction, turn him again. Continue until the horse stays at the desired gait. For obvious reasons you will want to do this before going on a big group ride as that would be a lot of pressure for the horse. Start alone or with another rider who will be able to stop and wait quietly for your horse. Then add the number of horses until your horse will listen in as big a crowd as you desire.
Hope this helps you.