Try this:
Mask off all all the area in the image except for the darker portion of the person...
Now, in the layers palatte,
A) - first, if you only have one layer and it's labelled "background", then double-click on it to make it a normal layer.
B) - Now, drag 'n drop that layer right onto the "New Layer" button. (see image) This will make a duplicate of the layer and place it over the existing layer.
C) - Now, for your new layer, goto the "
blending modes" drop-down box, and choose "
screen". (see image)
This should give a very natural looking lightening effect to the image. MUCH mo' better than trying use the dodge/burn tools
D) - If it's still not light enough, simply grab your new layer and drag 'n drop it on the "new layer" button. Another "screen" layer will be created and the lightening effect will be compounded, but should still look natural.
This effect can be done on the entire image (as my example shows) or if you use a layer mask on the screen layer then the lightening effect will only be in your designated area.
Just hollar if this still isn't clear and I'll (or someone here) will try to help.