When you got the bar on your back, before the first rep (I only do it for the first rep, not every rep), squeeze your glutes as hard as you can. For me, this seems to increase my glute utilization during the decent and rise of the squat. During this squeeze, I can feel tension released from my lower back. Perhaps this initial squeeze does a priming action for the glutes. Try it and let us know if this is the case for you too.