Julieanne Kost from Adobe has posted a great resource for converting color images to grayscale using Photoshop.

To use this template, first open and crop your desired image to 540 x 366 pixels at 72 ppi. Note: you can crop the entire image, or focus in on a smaller, more important part of it.
Flatten your cropped file and select "Save As" to save the cropped file as "FileNameFlatLowRes.psd"
Then open the template, click on any one of the smart object layers in the layers palette. Select Layer > Smart Object > Replace Contents, and navigate to select your cropped, low resolution, flattened file.
After the images are replaced, choose the best looking conversion, open your original high resolution file, and drag and drop the adjustment layer from the template to the high res file. Voila!

Be sure to check out her great site for more resources.

Great Black and White Conversion Resource