If its secondary to acne bellow advice will not help you...Tan is the skin's way of protecting itself from the harmful effect of sunlight (UV radiation).
Tanning is of two types - Immediate and Delayed. Immediate Pigmentary Tanning is the temporary darkening of skin that occurs on skin exposure and this lasts less than 24-48 hours. Delayed Pigmentary Tanning (also called a Permatan) lasts many weeks and if sun protection measures are not adequate this tan can be there permanently.
Treatment options depend on how motivated you are and how much sun protection you can practice with sunscreen lotion etc.
All of these measures are supportive - the real benefit to tanning comes from chemical peeling. 6-8 peels at 2 weekly intervals can improve the tan considerably. Microdermabrasion can be added to the regimen to further enhance benefit.