Probably nothing you aren't already aware of, but what you're experiencing is quite common for most people that don't have an O background. The wall and broomstick are good ideas, but another idea are dislocates. Grab a towel or broomstick with a wide grip,and with straight arms, let the bar down in front of you so it's on the front of your thighs, bring it back up with straight arms until it's overhead, then let it down with straight arms until it's behind you. The wider the grip, the easier the movement. Typically, with an overhead squat, the bar should ust be clearing your head by a couple of inches anyway, and in order to keep the weight over your center f gravity, your arms will be going slightly behind you. Dislocates, broomstick, and much practice3 is what comes to my mind. Anyway, there are currently several threads going on right now over at crossfit.com on the subject in the forums. Just jump in and look around in the "Exercises" section. Lots of good info over there from people more knowledgeable than I.