There are a couple other options... For a straight grayscale conversion without changing the color mode, try using a Hue/Saturation Adjustment Layer as your top layer with the Saturation set to -100. Takes all the color out, and can easily be turned on and off for quick previews. A slightly more complicated solution would be to instead of using Hue/Saturation, use a Gradient Map with the default Black-White Gradient as the basis. The gradient can then be managed in the Gradient Editor or a Levels or Curves Adjustment layer can be placed above it for smoother control.