I have been handling horses for more than 40 years. Much of my joy in life comes from horse packing and trail riding. I have often come across a couple simple mistakes many folks make with regard to saddling a horse for the trail.
Let me just make a narrative out of this. Firstly, a saddle should fit a horse as well as the rider. Some folks want you to believe there is a whole science dedicated to the fit of saddle to horse. When it comes to English saddles and Aussie saddles, there is something to fitting, but with Western saddles, which most of us use on the trail, things are pretty simple.
In general, when it comes to saddles, horses come in three sizes, small, medium, and large. That is normally expressed in gullet width. You have full-quarter horse bars, regular quarter horse bars, and semi-quarter bars. This generally means that the bars of the saddle are at about 92-93, 90-91, and 88-89 degrees, or thereabouts, from each other, respectively, depending on the tree manufacturer. I tell you that, so you know what they are talking about. For practical application, though, it is as simple as this: Without a blanket or pad, does the saddle sit pretty level on the horse's back? When you look under the gullet, is there about 2" of space between the horse's withers and the underside of the gullet (that's the part under the horn)? Are the rear of the skirts digging into the horse's flank? If the answers are "YES, YES, NO" the saddle fits. If the saddle is within one inch of the gullet hitting the withers, it will likely be very uncomfortable on the horse after more than a short ride. He/she needs a narrower saddle. If the gullet sits more than 2 inches above the withers, the saddle is pinching and will be uncomfortable on a long ride. Needs a wider saddle. If the skirts are hitting the horse's hip bone, the skirts are too long. Arabs and Arab crosses have short backs, as do some Paso Finos. They need shorter saddles. Of course there are individual horses that fall outside these general rules, due to injury or some unusual conformation, but for 99% of the animals out there, that's all there is to it.
You will sometimes hear of saddles for gaited horses. That normally means they have a semi-quarter bars and might have bars that are slightly twisted to accommodate a flatter back than the average QH. For the most part, though it's mostly a selling point and isn't a real issue. Contrary to popular belief, nobody makes a saddle tree to fit an individual horse. Even if you send in measurements or a back casting of the horse, the tree maker will just use those measurements to tell them which of their stock trees will best fit that particular horse. I guarantee that anybody who tells you differently is selling a very expensive tree.
Now for saddle pads. I often see folks with these thick, plush, fuzzy, pillow-like saddle pads. They think that since the rider certainly likes a soft chair, they are doing the horse a favor. Those type of saddle pads are like fishing lures. They catch more fishermen than fish. The best type of saddle pad is a simple 3/4 inch thick mohair pad, or wool pad, with a canvas or indian blanket top cover. The top cover protects and preserves the pad, not the horse. Thicker pads tend to make good-fitting saddles fit poorly and cause more problems than they cure. They also require the saddle to be cinched up too tightly, so as to keep the saddle from sliding around. Some folks who have been riding the range for years use nothing more than a wool blanket folded double. Forget the gimmick pads and save your money for a nice saddle. Keep the horse-side of your saddle pad clean. A horse can feel any little piece of grass or dirt against his/her hide.
Lastly, for this post, I often see folks cinching up way too tight. Strangely, the worst offenders seem to be women. I think maybe they are concerned about their weakness, but don't realize how that double turn of the latigo through the rigging and cinch ring multiplies their strength, like a block-and-tackle. Now this advice depends a lot on what you are going to be doing on your ride, but keep reading. If you have a decent saddle pad, as I described above, your cinch is tight enough when it is snug and you can easily slide two fingers, perpendicular to the cinch (not flat against the horse), between the cinch and the horse's ribs. Once you are mounted and have ridden a bit, you will probably find the cinch has loosened a bit. As long as the saddle is staying in place, no worries!
Here's how to properly saddle your mount. Always brush the horse first, to remove any debris or dirt where the saddle and cinch go. Lay the pad into place. Pat the pad and feel with your hand where the pocket in the horse's shoulder is, below and aft of the withers. Make sure you pad is far enough forward to allow about 2 inches in front of the front of the saddle's bars. Lift the saddle into place a little forward on the withers (not ahead of the pad) and give it a couple or three good shakes side to side. That will settle it right into place where it should be. Now check that the pad and saddle are centered sid-to-side on the horse and check the cinch again for dirt and debris (you should have checked it once before lifting the saddle into place) and pass it under the horse. While leaving the cinch loose, pass the latigo through the cinch ring, back up through the saddle rigging, and back down through the cinch ring.
Again, do this while leaving the cinch loose. If the horse is going to react to the cinch, he will normaly do so when it first starts to tighten on his belly. If you don't have those double wraps through the cinch ring, the saddle will go flying and your horse learns a new trick. Once you have the double wrap through the cinch ring, don't waste time. Snug up the cinch. Once it is snug, even if the horse takes off, the saddle will stay in place until you bring him/her under control. Now just let the latigo hang there while you bridle and finish prepping the horse. Just before you mount, give the latigo one more tug and tie it off. The cinch should be just tight enough that you can mount without the saddle sliding completely to the side. If the cinch is properly snugged and you mount properly, the saddle will probably slip a little to the left side. Don't worry about it. Just step hard in the right stirrup and give the saddle a little tug to straighten it. That's it. I have ridden many a mile on mountain trails with the cinch so loose my buddies could see air between it and the horse's belly when she stepped. Then again, she had good withers. I had another horse, a mustang, that was so round I had to ride with a butt strap and breast strap to keep the saddle from sliding up on his neck when going downhill! His cinch was a little tighter. Still, the cinch should be no tighter than necessary to keep the saddle in place. Cinching too tightly makes a horse "cinchy" and often leads to a horse that "bloats" or will not stand still to be saddled.
Now, I wouldn't recommend roping or cutting with a saddle cinched up like mine, but for trail riding it's all you need.