Everyone pretty much hit the nail on the head but 3 things you may also want to try are ankle mobility drills.
Someone mentioned this earlier but here is a link I have used.
If it is sort of a "fear" thing, you may want to try walkouts with around 110% of your 1RM. This just gets you used to having a heavy weight on your back and kind of gets rid of the "Oh shit" factor.
Also just sitting in a low squat position to warm up for 2 min. has helped me.
I like putting like 115lbs on the bar and getting down in an ATG squat position and just staying down there awhile. Also rocking forward onto each leg a little back and forth. Really loosens up the hips.
Sometimes I'm tight on one side and can see it in my squat. Really helps with that as well.
