Might try including some GoodMornings in your workout. Have aided many PL's as well as regular BB'ers when the lower back is the weaker link in the squat or many other exercises. Can also try supporting a heavier weight in your normal squat position and do 2" breaks at the knees. Tends to get the body set up to handle heavier, full squats in a normal workout. Training the ab's is another way to help and strenghten the core of the lower back/midsection. Strong ab's are very important in squating, DL's, etc.
Box squating is good, as Mr. Intenseone suggested. But you may have to have someone with a helpfull eye to watch how you preform them. It is a learning exercise. Or just start with the bar at first and go for 20 reps or so, just to learn the feel and style of the exercise. Sometimes just the way the bar is resting on the shoulders can have a bearing on squating style. High, middle or low on the traps can make a lot of difference for some people. Good Luck.
