A lot of big name designers think it's a great idea to look at your layouts in black and white to get a more clear idea of spacing, hierarchy and contrast. This is a quick way to work in monochrome in photoshop:
Go to the Layers palette and make sure you have the topmost layer selected. Click on the black and white circle at the bottom and select "Black & White" from the menu.
This will add a layer called "Black and White 1" that will make anything below it monochrome. Make sure this layer is at the top of your document or else it won't be of much use. Now you can show or hide this layer as you work and preview what you've done in black and white.