Dull skin is generally caused by a lack of oxygen and nutrients to the skin cells. This can be caused by poor blood circulation and tends to be a sign of aging. Make sure that you are hydrated, have a healthy diet, and are managing stress. Stress affects hormones such as cortisol which affects blood flow. Make sure you are getting enough sleep, and try to take a multi-vitamin everyday.
Exfoliation also helps prevent dull skin because it helps to get rid of the dead skin cells. There are are multiple types of exfoliators, such as acid and scrub exfoliators. Acid exfoliators help to dissolve dead skin cells, while scrubs physically buff off the dead skin cells.
Another issue that may contribute to dull skin is environmental pollutants, which is why it is important to wash your face twice a day, and use facial products that are high in antioxidants (and eating foods high in antioxidants is also important). Vitamin C is a potent antioxidant, as well as green tea, which make for good ingredients in facial products.