Watch your lines

Regardless of the exercise, make sure the muscle action comes from 1) mental engagement, focused consciousness of what is happening and about to happen to the body, and 2) quality, natural, elegant, coordinated, well-executed movements. 'Tis true for basic bodyweight exercises (bicycle crunch, bend & thrust, push up, whatever) to loaded exercises (KBs, squat cleans, deadlifts, whatever).

Notice in the pic, hips are back (posterior), and we need them vertical. How's that gonna happen? Drive the heels and use the hamstrings & glutes. You can almost visualize how this can go south...too much leg (squats), too much pulling with the lower back (ouch), too much shoulder movement and shrugging, too much up on the toes (knees). That's someone that's just approaching it as, "Muscles will move this." Instead of thinking, "OK, I gotta get this bar, straight up that line by driving my heels and firing my posterior side..."

Check out these 2 CrossFit journal teasers (subscribe in order to get full article). You get the point. Then watch the CrossFit video.
Deadlift Analysis
Squat -- Active Hip