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In a seated position, push downward with your lower back and rear end as though you were on a swing and pushing it forward. This helps you get used to the half-halt signal. When you're ready, practice on the horse in a walk: squeeze your seat and apply just a light pressure to the reins. If the horse halts, reward it by gently scratching him and praising him. Keep practicing this until the horse responds to the seat aid with minimal (or no) rein pressure. When you've mastered that, repeat those steps from a trot. If the horse is confused, you can return to the walk-halt exercise to reinforce the meaning of the signal. This simple exercise can help make the half-halt signal clear. In a way, both you and the horse are learning at the same time.  In a dressage arena, ask for a transition from trot to walk at A, E, C and B. Walk a few strides only, move back to a trot. Repeat this for a few circuits. The horse will quickly learn the pattern and expect to slow down at the given letters. Next, at those letters begin to ask for a transition to walk, but at the last minute change your mind and stay in the trot. This method lets you give a small suggestion of half-halt, and since the horse is expecting the signal, he is likely to respond. When you tell him to continue, he'll rebalance and move forward. Like the above exercise, this builds the horse's expectation of a change, this time working from a full halt. Try this if your horse doesn't respond to the exercise in Step 2, or use the two exercises in combination.  Signal a full halt and let the horse rest for two minutes. Trot about 40 yards (37 m), then signal another full halt, resting again for two minutes. After trotting another 40 yards (36.6 m), start your halt signal, but as soon as you feel the horse shifting downward, drive him forward.
Practice your seat aid at home before trying on the horse. Build the horse's expectation to halt through with a trot-to-walk exercise. Evolve your half-halt signal from a halt.