Some people asked me to repost this to make it easier to find on my blog, so I am adding this post to the "Photography" and "Tutorials/projects" categories on the left so that you can refer to it as needed.
This is the "Channel Mixer" method of changing a photo from color to Black/White in Photoshop:
Open your picture in photoshop and then open a new Channel Mixer adjustment layer.
Next, check the box that says "Monochrome" (on the bottom left corner of the box) to make the picture black and white. Don't click on "ok" just yet - there are more adjustments to make!
3. Next, you are going to adjust the brightness percentages to alter the colors in the photograph. With the channel mixer controls you can accent the brightness of the different colors in the photo by adjusting their percentages. To adjust the balance of colors, click on the sliders for each color and drag them to the right or left.
For example, if you increase the blue percentage, anything in the picture that is blue will get brighter.
Experiment and play around with the color adjustments until you get a vivid image that you like. The big secret is to make sure that no matter what adjustments you make, all three channels should always numerically add up to 100%. This will create the best results for the photograph.
It is really easy and a fun way to convert the image! Try it and see how it goes!