This is a great little tutorial about going from a rough hand-sketch to a clean, finished-looking rendering. There's a number of good tips in this, though I think the best one is the way everything gets organized and masked from the very beginning, before any actual rendering even starts.
The artist also references these other two tutorials:
For more information about "Layer Masks" in Photoshop.
For more information about "Pixel Locking" functionality in Photoshop.
Any chance to learn something is handy, right? If anyone has done any interesting renderings like this using these methods or any others, why not post some in the comments so we can tell you how talented and awesome you are?