i agree ponycrazy3, you do need to check for some sort of flinch, that is a common problem! although... is he alright with tacking up? i think the problem is most likely to be something to do with the saddle- do you clean it reguly? if not, do. because thats essential! is the girth OK? because the best ones to use are when you can get a furry soft sort of sock thing *( cant remember the name)* but its almost like a girth case; and i strongly recomend these as they make the girth alot more comfortable for the horse. 

is the girth the right tightness? it shouldnt be so loose its making your saddle slip, because not only will that proberly make you fall off- it also is very un- comfortable for the horse, can cause rubbing and sawness on his back. but the saddle should NOT be so tight he cant breathe! the correct tightness should mean you can fit two fingers inbetween his stomache, and the girth!

goodluck, really hope this helps! :) xx

Ensure all the usuals like back, saddle and teeth are OK and then persist that they are still.  Mine moves backwards as I mount so I make her move backwards ten more steps, re-position and then mount.  She eventually gets fed up and stands.

We have had a horse like this. He had a very sore back beacuse he was ridden by an unbalanced rider in a broken saddle, and taken hunting when not fit enough. The horse then makes the association that when you get on it's going to hurt, so even when the back pain is sorted out, you may need to reschool the thought, so that the horse re-learns that mounting does not mean pain. We did this very slowly over a period of weeks, with lunging to build up the muscles and lots of touching all over his saddle area - going back to basics really - with rewards (yes we gave treats) when he stood still. The horse now stands like a rock to be mounted, and also will 'stand' anywhere because the word means stay still and no longer has any bad associations - only good ones.

In short, before you think she's being naughty, make sure she is not in pain, then make the process as pleasant as possible so she has no cause to play up.