Just to chime in - most people when picking stuff up off the ground bend over. They bend the knees slightly first, then drop because it's less effort and only bend the knees more when they have to. Then straightening up again, they use the back first and then straighten the legs.
What everyone here is trying to get you to do is to sit back so that the hamstrings take most of the strain like they're supposed to, and then as you naturally squat down, you eventually fold at the hips and not higher up at the lower spine.
If you put your hands on your hips, and your index fingers along the crease in your hips, it'll move your focus to the hip flexion and off the lower back. Basically you should be sandwiching those fingers while able to keep your upper back relatively arched.
I personally found doing overhead squats made me do things properly. I suddenly felt where the squat was "supposed to go" and I haven't looked back since.
Oh yeah, for a great picture of great squatting form, check out Chad Waterbury's latest article on the home page.
Hope this helps a little more.