I've been playing around in Photoshop, trying to find a good way of adding a natural looking tan to a model. While there are a couple of ways of doing this, I particularly like this one.
1) Open your retouched image in Photoshop and duplicate it.
2) With the duplicate layer selected, go to Image -> Adjustments -> Black and White. Push back the Reds to about -100% and click 'Ok'.
3) Set this black and white image to 'Multiply' mode.
4) Create a new Hue & Saturation adjustment layer. When the setting box pops up, tick the 'Colorize' box. Set Hue at 44, Saturation at 24, and Lightness at 0.
(you can go back to this step and play around with Hue & Saturation settings to achieve the desired skin tone)
5) Apply the adjustment layer to the black and white image only (you can do that by holding the 'Alt' key, and placing your mouse between the adjustment layer and the image layer - you will see a little arrow appear - click it).
6) Place the black and white image and its adjustment layer in a group, then create a mask and invert it so that everything inside the group is masked. Set the group to 50% Opacity.
7) Using a large white soft brush, paint the tan back by unmasking.
Tip: watch out for eyebrows, eyes, lips and shadow areas - you might need to mask some of the tan back out from those areas, to keep the image looking natural.
It's very handy to make this whole process into an action if you think you might use this trick a lot - saves lots of time!