Here is a brief video of how to spot and correct a common deadlift mistake.
1. The hips should not pop up at the beginning of the lift; the shoulders and hips should rise at the same rate. If you or your trainee looks like Beyonce, then you're popping up too quickly.
2. When the hips pop it turns the deadlift into an RDL or stiff-legged deadlift which can a) place more strain on the lower back and b) doesn't tap into the awesome power of the glutes and hamstrings (and we all want some o' that!).
3. To fix: cue athlete to "pull chest to the back wall." It's akin to the "chest up" cue, but I like it better because it helps nail the shoulder-hip sequence more effectively than just cuing to pull the chest up. For me personally, it helps remind me to initiate with my glutes instead of my back and drive through my hips on the way up.
4. Deadlifting does magical things.