Possibly the most common mistake I see when deadlifts get heavy is the bar drifting away from the body.
Many lifters assume that a deadlift is just picking the bar straight up, but this isn't effective. As you pile on the plates, you need to think about actively engaging the lats and pulling the bar back into your body so that your weight shifts backward slightly.
Keeping the bar in tight helps ensure every pull is smooth and efficient.
Many lifters and coaches grasp the big picture when they read about the movements I prescribe for strength development, but they often miss the finer points. I've written this "Toolbox" series to help these experienced lifters fill in the blanks. It'll also help newer lifters learn about some very effective exercises.