It does tend to slip a bit on her, to the side (saying that, she was like a barrel, she was so fat, when I last had it on her). I have a pretty thick (also quite stiff) saddle cloth for under it, and it's not the most unslip material.
I only have a front cinch for it and I don't have a breastplate.
How tight should the cinch be on a western saddle? Any other ideas =/
EDIT: Also, the saddle was never used before I bought it, so it hasnt really been 'shaped'.