Biomechanics: Any advice for proper squatting technique? I have never been able to keep my knees behind my ankles.

The knees aren't supposed to extend forward past the toes. Keep your heels on the floor, hinge at the hips and stick your butt out to keep your spine in proper alignment.

You have not been able to keep your knees behind your ankles because it is impossible to squat keeping your knees behind your ankles. Perhaps you meant squatting while keeping your knees behind your toes. That too is borderline impossible for most people, although it can be achieved by greatly bending at the hip. At that point, the squat is no longer a squat but something that looks like a mix between a half squat and a good morning. This link: http://www.biomechfit.com/2012/02/09/3-squatting-myths-that-refuse-to-die is a good article with citations of the research papers that investigated the "knees behind toes" theory and found that although trying to keep your knees behind your toes while squatting somewhat reduces torque on the knees, it also vastly increases torque on the back. Your focus, while squatting, should be on maintaining a neutral spine, pushing your knees outwards to the sides and distributing your weight on the middle of your feet (with your feet flat on the ground).

Look at the olympic lifters, and see how much their knees extend beyond the toes. There is nothing wrong in knees going past the toes during squatting, that's all hogwash IMO.