Originally Posted by HitokiriChibi

I don't think there really is a way to do that. Best thing I'd suggest is to go over the pencil drawing in a fine liner, erase the pencil then scan it in to Photoshop. After that you mess with the Levels option, making the greys darker or lighter, the blacks more distinct and the whites brighter to make the paper look cleaner.

Or, if you don't want to ruin the original sketch - go over it in fine liner on tracing paper.

OR - try this tutorial I found on Deviant Art ^_^You'll have to go over it again and make the new line art in photoshop but if all goes well it'll look great.