How do I make a not-so-white background white in Photoshop?

[NicJ]

What is the EASIEST and cleanest way to whiten a background in PS. I know it would be best to get the lighting right in the first place, but sometimes that is not an option and the background comes out gray looking. What is the best way to fix that?

[OsmosisStudios]

Dodge tool: set it to affect only the highlights and be careful with it.

[Silverz]

use levels and set the background as the white point.
you may need to isolate the rest of the image depending on how the whites look there.

[TW0R]

A more non destructive way of dodge/burning also is to duplicate the layer, fill it with 50% grey, change the bledning mode to soft light, and then paint with white and black.