When I need to tan my skin for cosplay, I get professional quality brown theatre make-up and work small amounts well into my skin. It takes a while to get large areas of the body, but it does look pretty natural in the end. If you use too much, though, it won't look good, so you have to use it sparingly to get the effect you want. Using a moisturizer as a base will also help you to work it in better (Example
. You can contrast the fake tan with my actual skin color (the scar).)
When I'm lightening my skin, I use Revlon's PhotoReady liquid foundation in the lightest shade.