Assuming you are mounting from the near side:
1. Put your hand about halfway up the horse's neck and bury your
hand in its mane. Use this hand for pulling yourself up.
2. After putting your left foot in the stirrup, slide it forward so
your toe is resting against the left foreleg of your horse so your horse
can brace your weight agains his leg rather than having your weight pull
the saddle around the horse's body.
3. Do NOT put your right hand on the cantle at all.
4. Push yourself up with your leg on the ground (right leg) rather
than pulling yourself up with your hands and left leg.
I have successfully used this method of mounting a 16 hh horse into a
saddle with NO girth at all (to demonstrate how superfulous the girth is
if the saddle fits properly and the rider rides right).
Mostly it is step number 4 that keeps the saddle from slipping. In order
to do this, you will have to develop the muscles in your leg, hip and
lower back to be able to do this.
kat
Orange County, Calif.