For quick pics, I normally just go over the pencil with a felt tip pen, erase the pencil, then clean it up in Photoshop by adjusting brightness and contrast, as Tekno-Crystal said.
Pen lineartPen completeIf its something where I want the lineart to be very fine, I go over my pencil art in Photoshop using the Brush tool, usually at about 3px. The pen tool is better and more precise, but I don't use it because I'm not so good at it >.<
Using brush tool - don't have just the lineart uploaded for this one
Sometimes I can't be bothered outlining at all, so just use my pencil lineart:
Pencil lineartComplete, using pencil lineart