I recently discovered the program Sketchbook Pro and have totally fallen in love with it. Give it a try.
Here's some caricatures I did from late night TV watching:

I really gotta move away from Pete Emslie's style. It's a bit hard when he's taught you everything you know about character design.
And sci fi sketches for some job related stuff:

Generic Space Marine guy. Shoulder decal look familliar to anyone?

More mechs. Yes I know.