I cannot recall for the life of me where I read this tip, but I LOVE it. Symmetry is scientifically proven to be attractive. So it would only make sense to keep your eye makeup as symmetrical as possible. Step one: Get a piece of tape about an inch and half long and stick it to the back of your hand and peel it off again. This will keep it from sticking to your face too much and hurting when you pull it off.
Step two: Line up the tape so that the straight edge goes from the outer corner of your eye to the outer tip of your eyebrow.
Step three: Apply shadow and liner as desired
Step four: Hide from those who may not understand why you have tape stuck to your face. They will laugh.
Step five: Remove the tape carefully
Step six (optional): Use a medium stiff shadow brush and blend the edges until they're softened. If you prefer a more dramatic look, leave it as is.