Before I start painting something for a project, I like to warm-up by painting quickly and without worrying whether I'm going to mess it up. It's kinda like a shot of whiskey, it keeps me more relaxed on the next painting.
I like keep my warm-up piece somewhat related to what I'm painting for my next illustration, but I like to let my mind wander and do something fun. I never sketch these things out beforehand, so there's tons of mistakes in them as well as a few pleasant surprises. What you see is what you get. If I could do this painting again, I'd take that @!*# door out of the tree. Why did I do that?!?