More about tool modes.
If you want to make a background white, use your dodge tool and set its mode to highlight. I usually set the strength quite low, around 18 should do the trick.
The tool will now only affect the highlight end of the scale and not the shadows.
It works well with eyes also. Run the dodge tool over the catchlights and lighter parts of the eyeball to give them some sparkle. If it makes the parts of the eye that are suppossed to be black too light just use the burn tool set to shadows to bring that part back to where it should be.