Placing the bar higher on my traps helps me to actually go lower, it's not necessarily intuitive, as when you lean forward, it would place more torque on your back. But, it helps me to keep my back straight and keep everything in line. Since I've started going as low as humanly possible when I squat, the body mechanics of the squat on a whole has changed for me; for weeks I practiced with light weight, and now it's just natural for me.
As for hand placement, I find that a medium grip, so that your arms are at 90 degrees about is the best. I used to go wider, but one day I hurt my wrist bad while maxing out. Find the grip that is most comfortable for you and allows you to transfer the most power to the bar. Also, I like to use a thumbless grip.