I use a non slip pad under my saddle :) helps me all the time
Some saddle pads little velcro straps you can attach around your girth straps so it stays in place, and put your girth through all the straps and what not
Either use a non slip pad under the saddle pad or I'd use an ogilvy pad because they usually don't slip
I'd just spend the money and buy an ecogold pad. I used to have the same problems and then I bought one and my saddle nor my pad don't move. And they last a long time the first one I bought was 6 years ago and I still use it everyday since and it doesn't slid or anything. You can't even tell it's that old. And look to make sure that your saddle is properly fitted. That would help.
I had the Same problem (but with a jumping saddle), I idee a prolite pad, you have them in different sizes and different heights! They make your saddle fit your horse by for instance raising the front or the Back of the saddle, even both! You should inform yourself about the prolite, it helped me a lot even with different horses!
Chances are if your pad is slipping out that badly while you are riding(assuming you are on the flat and not on hills) your saddle is probably not fitting properly. BUT that is not absolute. I would suggest a wool pad, either full or half pad. Any of that synthetic material does not breathe at all and heats up the back muscles a lot causing a sore or sensitive back that can eventually cause the horse to loose their top line and not perform as well. Get the full/half pad that has the wool roll on the front and back that will also help secure the saddle.
I use one of the gel eze under bandages as a non slip pad. It works great! Although you have to rinse it off after every ride to keep it super sticky! Also absorbs shock as well!