I sometimes doodle characters with the idea of using negative space between them. Negative space in general makes things read more clearly and it also helps me with character design.
After I draw one character on a page I try to make the next one fit into the negative spaces of the other one's silhouette. The more characters I draw on the page the more challenging it becomes to fit a new character between the negative spaces of the surrounding characters.
This forces me to draw shapes that I might not otherwise think of if I was just drawing straight ahead.
Here are some HB style animals as examples:
..and some random animals
I've also been working on some beatnik mice. A friend suggested I call them "Squeakniks".