Hello..Thanks for your query
In order to prevent the darkening of skin, it is extremely important to use a sunscreen throughout the day.This should be applied 20 mins before sun exposure and repeated 4-6 hourly when outside.Even indoors it is important to apply sunscreen throughout the day.
This prevents not only tanning
, but also the other long term effects of sunexpoure.
You may also have oral anti oxidants and Vitamin C
to prevent photo damage to your skin.
Hope this is helpful