Warm up with sets of 10 reps, then have all your working sets be 5 reps. Glute-ham raises, leg extensions, weighted crunches, hyper-extensions and lunges will all improve your squat. Also start doing box squats, don't even bother doing regular barbell squats, guys with the biggest squats train their squat by doing ONLY box squats. Doing this will give you TEXTBOOK form and increase the strength of all the supporting muscles needed to squat ie abs, lower back, hamstrings, glutes. Also right down your weights each week, and progress by atleast 5lbs a week, and don't worry if you don't get 5reps, you can just build on it next week eg. week 1 315 x 3, week 2 315 x 5. Need anymore help just ask.