How is the imbalance affecting you? The viscera of the human body is not symmetrical, you've got a lung with an extra lobe on one side, which will affect the entire thoracic cage (and everything attached to it), and a liver on one side and a spleen on the other (which will affect which side I plant body shots on in the ring). Add a dominant side into the mix, and you've got a recipe wherein asymmetry is not only inevitable, it's normal
If you're asking from an aesthetic point of view, I gotta say I can't help you.
If it's affecting your lifts (any torquing, uneven depth etc) or you're worried about injuries, then we're addressing it as a movement problem, and I've got some suggestions. What you're going to want to look at is rotary stability, or the ability of your trunk musculature to resist rotation of the spine against resistance. I could direct you to the rotary stability functional movement screen test, but for rotary stability I find that the corrective exercises are tests in and of themselves.
Give these a try:http://youtu.be/XfuhB3Dp-BE
Along with some Pallof presses and some high to low chops. Find which side you have more difficulty with (if you have unilateral rotary instability, you'll feel a difference pretty easily) and do 2x as many sets on the weak side as the strong side for six week. In six weeks, reassess.