Thought I'd show a close-up of this cover and briefly discuss how I went about it. Yes, this is visually referenced from a photo of David Boreanaz that I kept on my other monitor. I don't know how to do likenesses otherwise. Tools: Wacom Cintiq, Photoshop CS3, and Painter X.
I usually begin every piece by very roughly establishing the values in grayscale and then, once the overall scheme is working to my satisfaction, I start adding color. After that, I blend the whole thing together with various Painter brushes, leaving kind of a blurry yet still textural mess. Even so, the overall structure of the piece is still intact and I can now go in and sharpen all the necessary planes and edges, making sure to leave alone the softer areas that already blend together well. Lastly, I polish/dirty it up with a wide variety of textural brushes to hopefully add some zip to the final art.
Still learning, but I'm beginning to get a consistent system in place.