Here is a sample dressage warm-up that I do with Brentina before a Grand Prix. You can tailor this type of program to whatever level you're training at, so this will give you a basic outline to adapt for your horse.
I walk Brentina on a long rein for probably 10 minutes, then pick up the reins and do a little shoulder-in and half-pass (if you're not up to doing the half-pass yet, you can adjust this warm-up to your own needs by doing a leg-yield). After that, I go to a rising trot, stretching contact down and round, doing variations within the tempo maybe 20 to 30 times during a three- to five-minute period. I'll take it forward for 10 strides or so and back for another 10. I will make several changes in direction during this period of warm-up.
Then I'll gather up my reins a bit and go to the canter from the sitting trot, bringing her back a few steps to see how I get her to react to my seat and legs, always focusing on the connection I have in my hand. If I feel there's not enough contact, I go forward and back a few times.
Next is a counter-canter through the short side, going back to the true lead across the next diagonal. I'll find a place in the arena, whether on the diagonal or on a serpentine loop, and change through the trot onto the other lead and repeat the same counter-canter scenario on the other side. Before I finish my warm-up, I want to make sure I can do canter-trot, trot-canter transitions and know she's reacting off sensitive aids before I give her a two- to three-minute walk break on a loose rein so she can relax, breathe and get oxygen to her muscles. Then I start training the movements I will perform in the ring.
If your horse has attention deficit disorder, you'll have to keep a little more contact with the reins during the break because you never want him to get so distracted that you have to start at square one again. It takes a while for you to find what works for your horse in a warm-up situation. After you do, keep it the same when you're in a show environment. That makes a horse feel a little more comfortable, even when he's in a strange place with a lot of distractions.
No one can give you the right formula for warming up your horse. You have to experiment and probably will need to try it at several shows before you know what might not be enough warm-up and what might be too much. You'll need to save enough brilliance for the arena. The one thing I do differently when warming up at a show is how I handle the extended trot. I keep that movement to a minimum for everyday work, because it can be stressful on a horse. However, before a competition, I want to make sure that the horse will stay with me and is secure, because it's one of the first movements in almost every test.
In Riding Through, Olympic medalist Debbie McDonald describes her system for success in dressage and relates her life story. To order, visit HorseBooksEtc.com.