Have you been working on an illustration that just isn't looking right? The best thing you can do to instantly see the problem is hold your drawing up to a mirror. If you are working on the computer just flip it in Photoshop. We tend to draw things more slanted than we mean to and often have more weight on one side of our drawing than the other.
Still not happy with your illustration? Take a break from it. Work on another drawing or take a walk. When you com back to that troubling illustration with fresh eyes you'll see your errors in a whole new light. I often work on three illustration at a time. If I get too involved in one illustration or start to fear making too many big changes I'll force myself to switch to another illustration.