A horse with a good set of withers will typically hold a saddle in place just fine, even on steep downhill sections. But if you ride or pack a horse or a mule with low withers, you will almost certainly want a crupper or a saddle breeching on the trail to help hold that saddle in place and prevent soring your animal.
Crupper
A crupper is a stuffed piece of leather, neoprene or other soft material that lays under the horse's tail. The crupper is attached to the back of a trail or pack saddle with a single or double attachment crupper strap and will keep the saddle from working forward. Most horses and mules take to this readily, but there could be a break-in period. So saddle up at home, attach the crupper and walk your horse around to let him get used to it before heading into the mountains. We prefer a traditional style, flaxseed-filled leather crupper. However, I have used a synthetic (Biothane or neoprene) with equal success and the choice between need only be dictated by your personal preference and experience.
To use a crupper on a pack saddle:
To use a crupper on a riding saddle:
Saddle Breeching
An alternative to the crupper is the Saddle Breeching. A breeching, sometimes known as a horse britchin, is a strap of leather that lays behind the animal's thighs and attaches to the saddle with two straps to the back of the saddle as well as two straps to the rigging. The breeching will keep the saddle in place on a downhill stretch without hindering the animal's gait.
Breechings are almost always part of a pack saddle rigging. To use a saddle breeching on a riding saddle, you must have two crupper/breeching dee rings of your saddle. A saddle breeeching will not work with a single crupper dee.
To properly fit the breeching: