Instructions
Respecting the Leg Aid
Ask your horse for a turn on the forehand. You'll do this by pressing one of your legs against your horse's side about two to three inches behind the girth and squeezing your hand on the same side to ask your horse to bend his neck. Only ask for a step or two at first, then let your horse walk forward. You can build up to more steps over time, until your horse makes a full turn. This teaches your horse to listen to your leg aids.
Press your leg harder, or even nudge or bump your heel against his side if your horse doesn't step his hind leg over. Correct your horse if he steps forward by squeezing both reins with more pressure.
Repeat Step 1 on your horse's other side. You always want to practice an exercise on both sides of your horse so he doesn't start to favor one side over the other.
Ride Forward and Back
Ask for a trot and, when you're happy with the pace your horse is moving, ask your horse to go more forward by pressing both legs against his sides. Again, nudge or bump with your heels if he doesn't respond at first. If he's still not listening, tap him strongly with your dressage whip just behind your leg (kicking will just make your horse resistant). Hold your hands still; feeling your horse's mouth is important to this exercise. Your goal is get your horse to take longer steps.
When your horse is taking longer steps, squeeze both hands and sit up taller in the saddle. If you're posting, post more slowly. This should ask your horse to take smaller steps. If he doesn't respond, turn him onto a small circle. If he slows down (rather than taking smaller steps), bump him strongly with your heels or tap him with your whip until he speeds back up.
Repeat the exercise until you are using less leg aids to get your horse going and less hand aids to get your horse to come back to shorter steps.
How to Train a Dull Horse
Dull horses, those that move slothfully when being ridden or are slow to respond to driving aids such as your legs, spurs or crop, are referred to as "behind the leg" by dressage riders. The key to getting these horses to be more responsive overall is to get them "in front of the leg." Dressage riders have several exercises they use to wake these horses up a little and make them more enjoyable to ride, and these exercises are applicable to every horse and rider.