One of the most common arm strengthening moves is the bicep curl. At any given point you can find someone doing bicep curls at the gym. Problem is, that at any given point you can also find someone doing them wrong by cheating. Let's identify a possible cause of the problem and figure out how to fix it...
How to Identify a Cheater: Bending the upper body forward (or backward) at the waist makes the bicep curl a lot easier (and more harmful to your back, mind you) because the swaying allows momentum to do the work when it should be your biceps doing most of the work.
Rehab: Stopping your upper body from swaying forward and backward is the key. To do this, stand against a wall and make sure your back is in contact with the wall throughout the entire movement. You can also try sitting on a bench with a back, but don't push through your legs!
Fit's Tip: If you find that you still want to sway, even when backed up against a wall, then chances are that you're using a weight that is too challenging for you. Lower the dumbbell weight and work on your form.