Does your horse have noticeable withers? Those that come without saddle anchor points are tougher. I had an Arabian mare who had withers, but they ended about where the saddle started, so not much luck. However, she didn't have a round back either.
Some other things you can consider...use a different type cinch. Some 'grab' the bottom of the horse's chest better (this one is good for that, IMHO Professionals Choice SMX Roper Cinch - Statelinetack.com
). Change the pad under the saddle. I like wool felt, but it doesn't grab as much as some others do. This one has more grab to it (Reinsman TackyToo Microsuede Saddle Pad - Statelinetack.com
You could also look at mounting techniques. A mounting block can help a lot. Also, make sure you are close to the horse, and get your weight centered over his back ASAP. If you are further away, it puts more torque on the saddle. As soon as you are high enough, lean a bit over the back so the weight is balanced on top of the saddle and not on one side.
Also, I've been taught that my left hand goes in front of the withers, and my right hand on the rear of the saddle, so that all the weight doesn't concentrate at the cinch. Hopping a bit as you are about to mount can help a lot too.
The two horses I now own both have long, tall withers. I swear that if I ever buy another horse, he'll be no more that 14.2 in height and have good withers.
Here is an oldie but goodie thread on mounting techniques: Mounting using the horn
Editing to add that cinching down with the force of Thor isn't a good technique. I've done it, but my horses were happier when I used a good fitting saddle, the right pad & cinch, and good technique for getting myself off the ground.