All human skin reacts to sunlight by darkening unless the skin is naturally of the very darkest sort. Avoid the sun, and any other source of ultra violet light, the main cause of this effect, and your skin will become lighter.

If you can organise your life to avoid light falling on your skin you will not only stop the suntanning process but any pigment in your skin that is a result of previous exposure to sunlight will fade - this may only just be perceptible unless you have a strong tan in the first place, as it does take several months.

There will be detrimental effects to your health if this is taken to extremes as beneficial vitamin D is created when sunlight interacts with skin. A lack of Vitamin D, is directly related to bone health, lack of it can cause osteomalacia (rickets).

On the other hand too much exposure to the sum causes premature ageing in the skin, increased risk of skin cancer and can reduce the effectiveness of the immune system - amongst other things.

Once your skin has reached it's natural colour there is no proven natural way to lighten it on a permanent basis.