Mounting properly can fix this.
I never thought about it, until I trained with this one die hard dressage clinician who raves about proper mounting. You should not put any weight in that left stirrup. You should be pushing off with the right foot, and with one hand on the mane, lightly boost yourself up. Takes a little practice, but it's not too hard.
Since then, I have bought a new western saddle, and the leathers have stayed perfectly even.