Most Helpful Girl
I think confidence is the key in all situations. Just look good, well turned out and wear clothes that go with your personality and that you're comfortable in. Don't look NOT like you because you'll be uncomfortable and you won't feel as confident either. Your personal style should reflect in whatever you wear because that's what sets you apart from the rest and he'll notice that for sure.
P.S. Please don't make him want you back.. if he didn't realize what he had when you were with him, looking hotter now or dressing better won't change the fact that he left you to begin with.