I've read that many artists start each painting session with some exercises to get themselves into a "painter's mode". I've taken to grabbing a book of Man Ray celebrity photos and just doing a quick drawing. It makes me practice more realistic work and calms me. I still end up working on mostly abstract and/or non-representational stuff but the structure of the portrait seems to be a good starter. I take 5, 10, maybe 15 minutes.
This one is of a young Ernest Hemingway. That left ear is a problem. ;-)