One coaching cue I shameless stole from . . . someone, somewhere, is to "stand tall." At the top of a lunge, a step-up, a squat, a deadlift, during a press - you always want to "stand tall." What does that mean? Proper erect posture, with your head facing straight ahead, shoulders down and back, chest "open", and your glutes locked.
What's nice about "stand tall" is most people have a strong mental image of standing up tall like this. Cue them with it, and they'll stand properly at the top of a step-up (no lean or hunch or looking at the feet), they'll come up straighter out of a lunge (instead of making it a lean-over lunge), and they'll activate their muscles properly.