First, learn how to sit it yourself if you want better, quicker results, on a horse that is already trained to do so. I learned it just from lots and lots of riding on my gelding who is bred to have nice western pleasure gaits, not that he always does them, but he has the talent naturally. I use a regular snaffle, and lightly put pressure on one rein at a time, rhythmically, and lean back and say easy. Do this until the horse slows down, then let them on a loose rein. Once they get it, move on to the trot and do the same thing. (You can play with the walk until they are just barely poking along.) You have to be able to sit the trot when teaching them, no matter how fast they are going. Don't hold hard on the mouth, use your seat, and be clear with your commands. This is the method I use, but I've also heard of putting your horse out on a circle, and if they go fast, pull them into smaller and smaller circles until they slow down then letting them go out in the bigger circles. If they get fast again, pull them back in. Keep repeating until they figure it out. I've seen it used effectively, but I've never done it. If you're really interested in learning more, take some lessons and have fun with it. It's always nice to venture into different disciplines every once and a while. Really diversifies your riding abilities. Right now I am looking to start dressage lessons, and I think I'm going to in for a work out, but it would be worth it to improve myself. Good luck, and happy riding.