Switch from a squat rack to a leg press machine. It's a lot easier to make sure your back is in proper alignment and KEEP it there on the press. Plus, you can do a lot more weight (that your legs can actually handle easily) if you are not trying to hol dup the weight in a squat rack.
For stretch, you'll look like a ghey *** doing them, but the "cat stretches" help limber up the lower spine and make things less painful. On hands and knees, roll your spine up like a pissed cat. Then roll down and stretch your ass up into the air (the gheyest part) Keep the abs tight all the way through.