We generally use the Channel Mixer (as an adjustment layer) to convert the individual color channels in an image to grayscale values. The most important settings to make are:
Activate the Monochrome option (Figure 10-2). The sliders are then automatically set to +40, +40, and +20. These are the default conversion values.
Then use the individual RGB sliders to determine how much each channel contributes to the resulting grayscale image. A good rule of thumb is to set the values so that they total 100%. More than 100% results in a brighter image, and less makes the result darker. As of CS4, Photoshop automatically displays the total at bottom right of the dialog window.
Check the individual channels ...