One of the trickier parts of the Olympic lifts is the first pull from the floor. Depending on the athlete, some may do fairly well from the hang position, but as soon as the bar goes to the floor, the pull becomes a mess. Sometimes it's failing to hold the back angle (in relation to the floor), sometimes the hips pop up too soon, for others the shoulders drop back behind the vertical line of the bar.
One cue that I have used with pretty decent success is to 'pull like a pole dancer'. This seems to help clean up the first pull by getting the athletes to hold their hips back (butt out, against an imaginary pole), while getting both the shoulders and hips to rise at the same rate. Sure the hips sliding up the pole my be exaggerated for some, but cues that get an athlete to exaggerate the movement usually make it easier to settle into a happy medium with what you are trying to accomplish.
**I strongly suggest having a great relationship with your athletes first, so as not to be offensive in using such a cue.