Besides all the other great advise you have gotten, the only thing I can think of is the cinch is too long or short. In my experience if there is not enough or too much space between the buckle on the cinch to the rigging on the saddle, the saddle will slip no matter what type, saddle or cinch you have. Same if it is too short, it then pinches and you will get the slippage.
The cinch buckle on the cinch after it is tight should be above the horses elbow
ETA- Your saddle is going to slip some when you get on. Especially if your not too graceful like me
I use a mounting block when I can, just for that reason.
You do want your saddle loose enough so that if you need to adjust it from in the saddle you can do so by stepping down on one side or the other of the stirrups. If when you do that it slips so badly as to swing around than, yes its too lose. Otherwise its about right.
What are you grabbing when you mount? I grab mane with one hand and pommel with the other. If your grabbing the back of the saddle that will make it slip too.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France
Last edited by Vidaloco; 08-03-2009 at 04:56 PM.