Leaning on the bit is the common description for any horse that puts an undue amount of its own weight on the bit while being ridden. A horse that leans on the bit will typically cause its rider quite a bit of muscle strain and discomfort because the horse is, in fact, using the rider as a way to support and balance. Leaning on the bit is a bad habit that takes time, conditioning and a certain amount of rider experience to break.
Determine that your horse is leaning on the bit because it is lazy or bored.
A horse that leans on the bit for these reasons is typically going to be an experienced, well-broken horse that is used for basic, lower-level riding. There is little challenge in the manoeuvres for the horse and so it settles onto the bit and chugs around the ring or down the trail, making the rider support part of its weight.
Lesson horses or those ridden often by inexperienced riders have a particular knack for leaning on the bit. Inexperienced riders typically don't have the knowledge or skill to do anything about it. Some of these same horses will get off the bit very quickly when mounted by an experienced rider with the knowledge base and skill to make the horse use its body effectively while being ridden.
Learn how to do a half-halt.
The half-halt is a subtle gesture where the rider will slightly tighten the reins and then release them as the horse hesitates slightly. The half-halt can be used to get the horse's attention and moderate its movements. You can repeatedly half-halt a horse to get it to stop leaning on the bit, but half-halts are most effective when combined with other exercises to thoroughly engage the horse in the task it is supposed to be doing.
Half-halts should only be done by riders who already have a reasonably steady seat and hands and can perform the cue without jarring or jerking on the horse's mouth.
Challenge the horse.
If you are an intermediate rider or higher, you will probably be able to get the lazy horse off the bit by making it perform more complex manoeuvres that require it to pay attention and balance its own weight effectively. Combining half-halts with circles, figure eights and other various bending exercises done randomly will force the horse to pay attention. Bending is a simple fix for a relatively fit horse that wants to lean on the bit. A horse that is moving forward at a reasonable pace and bending properly in the corners of the arena will not be all that inclined to lean on the bit.
Modify the behaviour with speciality tack.
Inexperienced beginning riders may not be able to effectively correct a horse that leans on the bit. In this case, the behaviour may be modifiable with speciality nose bands, bits that are harder or less pleasant to lean on and possibly a martingale that can be adjusted to where the horse is leaning on itself instead of the rider.
All of these options require special individualised assessment of the problem and the potential solution by an experienced rider or instructor. Beginners should not attempt to correct a problem by making tack changes without the input of experienced professionals. Poorly selected or ill-fitting tack can cause problems significantly worse than leaning on the bit.
Determine that your horse is leaning because it needs to, not because it's being lazy.
Young or out of shape horses will lean on the bit because the horse has not figured out how to properly balance its weight while under saddle. Young horses and out of shape horses, will lean on the rider for additional support while performing faster or more difficult movements.
If you release your reins in a turn and your leaning horse stumbles, there is a good chance the horse is not balanced very well and using you to compensate.
Develop a conditioning program.
In order to get a young or out of shape horse to stop leaning on the bit, you will have to help the horse develop the muscle and balance it needs to support its own weight without using the rider for extra balance. Your condition program should include regular riding, strengthening and bending exercises.
You will need to get in contact with an equine professional in order to work out what exercises will be most beneficial to your horse's individualised needs.
Teaching a horse proper balance and building conditioning are not overnight processes. You will need to devote regular time and work to training your horse out of this problem.
Training a horse should be done under the guidance of a professional. Improper training methods can cause a number of larger problems over time.