A stiff belt and a stiff holster with at least two attachment points will help. You can also hold the belt a bit tight with whichever hand is free. The deeper the holster rides, the more stability you'll have, but sometimes it just doesn't matter and stuff is falling all over the place. If you're wearing anything less than a stiff, quality belt, none of the other variables matter. If you're serious about carrying and do not have a nice belt, get one and you'll thank yourself three times a day until you don't even notice anymore.