Stretch, stretch, stretch....triceps, wrists, foam roller on your thoracic spine, lacrosse ball dug into the nasty bits of your shoulders and scaplulae. All of that can help, because lack of mobility in any of those areas can cause difficulties with the rack position. hystikal is 100% right, be very wary adding weight until you can do the moves correctly.
Really practice a nice air squat with your chest up and your spine neutral too.