Unfortunately I'm an English rider so I don't know if any of the following will help but I'll just run through what I did to slow my pony's extremely fast trot down and maybe you'll be able to use some of it:
Half halts: These usually work extremely well, close your leg on your horse and check with your hands which should get your horse together. Unfortunately these did not work with my horse as when I closed my hands, she simply put her head into the air and ran faster. I've been told gently sponging with the reins can also work with some horses, again, not with mine.
Trotting poles, barrels etc: I found trotting my horse over poles/around barrels etc helped to get her concentration and it was easier to slow her down and get her to focus.
Voice aids: This worked well for me, teach your horse a voice aid which means 'slow down', use 'woah' or any word that suits you.
This might not be accurate dressage wise, but I found closing my thighs on my horse in combination with slowing down my rising trot and closing my eyes to take a big deep breath helped in slowing her down. For my pony, I had to 'train' her that slow is what I wanted. So I would do all of the above, then as she responded, I gave her rein, let her stretch down and relax and then brought her down to a walk. So she would only be able to stop working once she gave me a nice slow trot. Since then I've been able to work her at a slow trot but gradually increasing the amount of slow trot I wanted before she got the reward of stopping work.
Now she has a nice slow trot in a nice, relaxed low outline.
Last edited by munschk; 04-10-2010 at 12:44 PM.