A good halt should be considered not only as coming to a stop but as a movement in itself. In a good halt the horse not only stops moving but stands with his weight taken evenly on all legs and with a leg at each corner - this is known as a 'square halt'. To achieve this the rider must be riding straight up to the exact moment of the halt.
To practice this it can be helpful to make yourself a corridor of cones to ride down, coming to a halt in the middle of the corridor. First make sure you have a good forward going walk as like any other transition the halt will depend largely on the pace before it. A good halt should not be a loss of energy but rather a containing of it. You should feel that even though your horse is motionless it could spring forward again easily.
Like any downwards transition remember to prepare your horse before coming to a halt. To prepare for the halt begin to block the walk with your seat by moving less fluidly with the horse. Think of holding your body a little more rigidly and using your abdominal muscles to 'brace' a little against the pace. Begin to feel the tension on the reins allowing your hands to stop following the full nod of the horse's head. At the same time tighten your thighs and knees a little against the saddle. This preparation stage should be very subtle and the horse should not slow down but the intention is that the horse gathers itself. You can think of this as a half halt; remember that the idea is that horse does not lose energy.
When you are ready to halt apply a very slight pressure with both legs whilst increasing the pressure on the reins and blocking the horse's movement with your seat. Try to feel how your body helps the strength of the rein aid, as this will allow you to use a firm aid without just pulling back on the reins. In order to achieve a square halt all aids must be given symmetrically. Using uneven aids tends to cause the horse to turn as it slows down and halts. Keep gradually increasing the rein pressure until the horse comes to a halt then release the tension in rein and body. Your legs on the horse's sides throughout will ensure that you keep the energy.
Achieving a square halt is mostly a matter of preparation for the halt and learning to use your aids evenly on each side. Right handed people may find they are unconsciously pulling harder on the right rein and vice versa. Try to be aware of what you are doing and if you find your horse always halts crooked to one side then try softening your aids on that side.
Once you are in halt feel how the horse is standing. You should feel that both your hips are level; if one hip is dropped it is likely that your horse has left that hind leg out behind him. At first practice feeling what you have got, decide how square you think your horse is then look down to see. (Or better still have someone tell you!). If you were wrong think about how it does feel, then walk away and try again. When you can tell how your horse is standing think hard about keeping that straight feeling throughout the transition and into the halt.
You can make some corrections once the horse has halted. For instance you can move the horse's left hind leg up by squeezing with your left leg, whilst keeping the forehand still with your seat and reins. However, you should only make small corrections this way otherwise the horse tends to start fidgeting as soon as you halt, anticipating your fiddling! It is often better to simply walk on and try again thinking what was wrong with the halt and correcting it in the next one.
Bearing in mind that a horse moves away from the leg aids you can use this to balance your signals to affect the horse. For instance if the horse falls left or leaves a left leg behind him use your left leg a little stronger coming into the halt.
Achieving a square halt is desirable not only in the dressage arena but it also results in a better upwards transition as you walk or trot away.