Answer

By Your Horse

Dressage Rider and Trainer Claire Lilley advises:

To get your horse working properly will mean lots of transitions, making sure that he uses his back correctly at all times. Stretching work is just as important, which can be done on the lunge.
If he has an over-developed muscle on the underside of his neck, which he may use when he becomes strong, a chambon will help him to use his topline muscles and relax the underside of his neck. At the same time, he will learn to use his belly muscles to support his back, making him more able to work with his hindlegs taking the weight, which will lighten his forehand. Pole work is very useful for the same reasons, making him more athletic and supple. Then he should not be so reliant on the reins for support, and be able to accept a lighter contact with the bit.
In the saddle, it is important that your riding position is correct, and that your half-halts are effective and correct. You will need strong stomach and back muscles to do this, and be able to use your thighs, knees and calves correctly, to keep your horse working in balance.