The squat is my favorite exercise. It has been from day one. I have been lifting heavy weights for over 20 years and the squat has been the core of my exercise routines every step of the way.
There is nothing like the feeling of burying a heavy squat, exploding out of the bottom and pushing up some serious weight. If done properly, the squat can be a total body lift. It is not just for quads. Nothing, except maybe the dead-lift, is going to put on more mass and strength and make you more explosive than heavy squats done with proper form. So let's get at it!
The most import thing when doing a squat is NOT to let the bar drift forward ahead of the knees. If you drew an imaginary line from the bar to the floor, this line would never come forward enough to touch the knees. Sit back into it. Lift from the heals, not the toes. Let the weight fall into your hips and your butt and your upper quads. Don't let the weight come forward into your knees and lower back.
This may sound kind of weird, but if you had a bobber hanging from a string and that string was hanging down from your butt, that bobber would never move. It would not swing back and forth, it would just go up and down.
You can play around with the positioning of the bar on your back and your foot placement. Put the bar higher on the back and put your feet closer together to isolate the quads. By putting the bar lower down your back and having a wider stance you bring more muscles into play -- your butt, hips and upper back. Obviously you are going to be able to do more weight with the latter set up.
Squat at least to parallel if you can. If you drew a line from your hip joint to your knee, that line should be parallel to the floor at the bottom of the movement. Remember, a fuller range of motion (as long as you maintain proper form) creates more stimulus on the muscles and it increases flexibility.
I think it is funny when a guy brags to me about how much he can squat and then when I see him lift he is barely even getting down to a half squat. Lower the weight, use proper form, get down to parallel and then over time increase the weight -- doing a real man's squat!