Hummm, what comes to mind first, is the saddle does not fit the horse. Too wide maybe? If you want to see how the saddle fits, get a white sheet, put it on the horse, and then put the saddle on without the saddle pad. Cinch up, and get up in the saddle. Then get off, and see the dirt pattern. Turn sheet inside out, and repeat with the saddle pad.
Also can take a coat hanger and bend it to fit the horse then see how it fits into the saddle. Some horses need thick pads, and some need thin pads, but either way the saddle needs to fit.
I ride western and I have a tex tan flex tree, FQHB. It fits my mare well, it never falls or slips unless we are going up a steep incline, then it will slip just a tinsy bit back, but no more than 1/4 inch without the brest strap. I use the brest strap to take the strain off the girth, not because the saddle slips.
I use different girths, I have a fleece one and a neoprene one. I hope to get a rope one. I use lether cinch straps. If the holes in the cinch don't match up properly, I use a cinch knot. (Actually I use the knot anyway, as I don't totally trust the holes...)
This site has instructions on how to check for saddle fit. http://www.horsesaddleshop.com/allabsadands.html