This one is easy. Dull skin is either poor facial nutrition (general or local) or a build up of dead skin cells on the exterior of the skin. If you notice your skin getting dull, look at your skin care routine first. You may be using something too harsh on your skin, not using good nightly cleansing practices or using products that are causing an allergic reaction. Then look at your diet. Perhaps you are not getting enough antioxidants.
I recommend a Remove the Dullness facial to jump start your efforts to a more vibrant skin. This is usual a one or a sporadic professional facial responding well for long periods of time. I would match this with an at home treatment which includes a cleanser that matches your skin type from the Eminence line or Mangosteen Cleanser with a cleansing machine I-Sonic or Clarisonic. Rinse off the Cleanser with a wet, warm washcloth. Follow with Stone Crop Hydrating Mist, Mangosteen Concentrate, a Sunscreen/Moisturizer to your skin type for day and Lotus Overnight Detoxifing Treatment at night.