clean, straight anti squel shims. maybe a touch of anti squeal stuff (copper, goop, whatever)
clean calipers, pins, springs, clips. dust allows movement that causes squeal
straight, unwarped discs
properly bedded pads.
I've used probably 10 different types of pads including very aggressive race only pads, and none of them squeal when installed properly except super aggresive endurance racing pads.