When I do sketches for an art director, I have to draw them neatly so someone else will understand what my final intentions are. For my cards however, I am the art director and the drawings themselves tend to be pretty scribbly. I find this very freeing and it is always my starting point for new artwork. They are purely the idea and only to help me start thinking in pictures. As you can see from my examples, my thinking is pretty fuzzy at first, and then gets clarified as the artwork progresses.
Here are a couple of pages from my sketchbook that show my ideas for this years Valentines cards.
After I've made some little drawings I like to make a few written notes about color ideas or drawn details. There are always a few ideas I don't use, but sometimes I do later.
Here are the finished pieces:
