The first thing I do is to try and not bring what I probably won't need with me. Then, I constantly push all the gear I do end up carrying around to rest on my back. My harness is a Cadillac, so I use the large upper loops as a "stop" to hold the gear hanging from my sling in place. Some days my efforts work better than other days.
This problem gets better, but your gear always finds a way to set itself now and then. This is part of what makes trad climbing "fun".