Stay Out of Valgus
While there's been a lot of debate about shoving the knees too far out, most agree that pushing them too much inwards isn't a good thing, at least if you want to keep your knees healthy. This inwards-knee thing leads to what's called a "valgus position" and leaves the knee joint vulnerable, especially under a heavy squat.
This is one of the most common squat problems, and while many point to weakness in the quads as the reason, it's not necessarily so. When you look at the attachment of the quads and how they work, you see they have little effect on how the knees move from side to side and in rotation.
Most of the time, the knees moving inwards have to do with either weak hips, and/or lack of proper technical understanding of what's supposed to happen during a squat movement. Furthermore, when you see knee problems, you almost certainly will detect hip problems as well.
How to Fix It
To remedy this, train with some bands around the knees to make it easier to activate the hips during the squat.
You can also do banded walks or hip abduction exercises.