Most artists have sketch pads or drawing journals (or several hundred!).  I call them creative junk draws.  You know that draw you have where you collect a thousand odd bits that you know might be useful someday. 

 

My stuff usually start as good old #2 pencil drawings, then sharpie over the lines I like.  For color I tend to lean on markers and colored pencil, occasionally chalk pastels and acrylic paints... even some white out.  Finally I may import them into the computer and tweek them with some techno magic.