I like using a hue/saturation adjustment layer. That combined with layer masks makes for ultimate control (very easy to go back without having to redo a bunch of steps--you shouldn't rely on your history pallet to be there).
These might be a little advanced if you're just starting off, but I highly recommend learning them if you want maximum editing power. As an example...If you add a layer, desaturate, and erase the parts you don't need, what would happen if you wanted a different part in colour a week down the road? You'd have to delete the current BW layer, make a new layer, and go through the process again. Using adj. layers and masks, you wouldn't have to delete anything--simply colour in the parts you want to hide/show.
Everyone's methods work, but having had to go back into working files after the fact (one was more than a year old), I'm glad I added this flexibility into my files.
Lastly, it should go without saying that you should never save over your original. Right?