i need help cleaning up a graphic drawn in pencil. i'd like to make all the lines cleaner, straighter, and in general make the graphic look more like it was created digitally. can anyone tell me how to do this in ps or gimp?

You can use leveling to clean up a lot of gray areas

This tutorial is about photographs, but talks a bit about leveling.
There's also a lot of guides about cleaning manga that may be helpful, like this one. Also some good deviantart ones.