To avoid causing injury to your horse, you should check the fit of the harness every time it is used and adjust if necessary. If you are unsure how to harness your horse, it is best to consult your veterinarian. An ill-fitting harness can cause painful injuries to the horse. If you notice any weak spots or problems with your tack when harnessing the horse, repair or replace it immediately to avoid potential problems.
Make sure that the horse's eye sits at the middle of the blinder if you are using blinders. They must be close enough to the horse's head to ensure that he cannot see behind him, but the blinders should not touch the eye or the eyelashes.
Secure the throat latch only tight enough to prevent the bridle from sliding down the horse's face. You should be able to easily slide your finger in between the throat latch and the horse. The throat latch will cause the horse to choke if it is too tight, so check this every time you harness the horse.
Prevent sores on and pinching of the lips by checking that the bit is not hanging low enough that the horse can get her tongue on top of the bit. Tightening the nose band will keep the bit where it belongs. The nose band should be snug against the horse but never tight enough to cause indentations on the skin.
Double-check that the bit has not been pulled too high into the horse's mouth.
If the bit you are using has a curb chain, make sure that you can easily slide two fingers between the chain and the horse's chin.
Ensure that the breast collar is loose enough that it does not interfere with the horse's breathing, but sits high enough that it does not impede the animal's shoulder movements.
Adjust the belly band and girth so that you can slide one finger in between them and the horse. The belly band and the girth should not be as tight or as far back on the horse for a harness as they are for a saddle.
Ensure that the back saddle does not actually touch the horse's back. There must be space between the horse's back and the tree, and the spine should be centered in between the tree bars. It is crucial that this is done properly so that there is no direct pressure on the animal's spine!
Properly adjust the breeching strap so it acts as a brake for the carriage when the horse is going down a hill. The breeching strap should sit snugly at the very bottom of the horse's rump. The strap must be low enough that it does not interfere with the tail dock but should never be low enough to hit the hock. The horse should be able to comfortably lean his haunches back into the breeching strap.