The best thing to do is start and aggressive belaching regimen along with strict sun protection using physical block sunscreens. I like Obagi. You should check with your docor so he r she can determine whether you have postinflammatory hyperpigmentation or melasma. If it is melasma, then you will need to stop birth control pills and other hormone therapies if you take any.
It is rare to have prolonged hyperpigmentation after Fraxel, but it can happen. The best treatment is a combination of Retin A/Hydroquinone (we like to use 10%)/ and Hydrocortisone cream. This is usually very effective.