This video tutorial by portrait photographer Nathaniel Dodson (from the website tutvid) takes a look at color grading in Photoshop.
This video is also part of a longer video series that Dodson is making describing his retouch process.
What Dodson shows is how to control the curve adjustment layer and play with the different color channels by adding points and very carefully controlling the colors. By adding a color adjustment layer you are also faced with the option to change the highlights, midtones and shadows for all the colors (again be careful – too much can really make your image look unnatural).
The final method to color grade is by using a gradient map – we select the color that we want to go to the darkest pixels and the shadows and set the blending layer to soft light.
Do remember to reduce the contrast if you use this method because it tends to increase the contrast (sometimes a bit too much).