If your saddle slides, you are experiencing one of two problems.
Problem one, your saddle does not fit correctly. The only solution to this is to have it fitted by a reputable, accredited saddle fitter. This may not be possible, depending on just how BADLY the saddle doesn't fit.
Problem two, your horse is roly-poly, with no wither to speak of, and no matter what, any saddle is GOING to slip. There are a few solutions to THIS problem. You can get non-slip saddle pads (or make them, it's cheaper), breastplates, special girths and cinches... there is a solution for everyone.
Believe it or not problem one is the most common. I had a very wide, very roly-poly horse that every saddle slipped on no matter what... until I bought a saddle that DID fit. Turned out, I wasn't as good at fitting saddles as I thought I was. Lesson learned, all my saddles are now professionally fitted.