Thought this might be helpful.
Many of us use curves and levels on our images. But there is one particular technique using curves that can do wonders - like removing cast, adding punch and optimizing levels in one go.
Here are a couple links to get started:
Tutorial on S-Curve technique
And some S-Cure presets to download - very handy when your starting out.
Another thing to consider when using curves or levels is the eye droppers for selecting Black, neutral and white areas of your image for greater control. Something to once your familiar with curves and levels.