Run your fan on the lowest level and watch it spin to identify which blades are causing the fan to go off balance.
Tighten the screws on the drooping blade, as loose screws are the most common culprit. You'll find screws holding the arm on and screws holding the blade to the fan's motor.
Check for bent blade brackets. Gently bend the bracket back with your hands or a crescent wrench. Place a level on the top of each one of the blades until the angles match on all the blades.
Check for damage to the blade itself. If so, replacement blades are available at most home-improvement stores.