Try playing with your stance and depth. I've narrowed my stance and started doing a rep that is just shy of the top and bottom. Once you get the rythmn going the burn is excellent.
Also try switching the order, squats kill after you have already done some high rep leg presses, easy to get a great feel for the movement.
King Komali talks about doing squats with a very light weight, 145, and going for 30 - 45 minutes straight. Not sure if I'd want to but you could vary it.
I've always found once a workout gets stale I just have to switch the order, reps or weight and things pickup again.