For those of you who wonder about messing up, worried working from memory or just doodling, this is for you. This is the kind of stuff I do while either sitting at work, sitting in the passenger seat of a car or when boredom is having its way with me (which tends to be more often than not).
I draw whatever shape comes to mind (turning that into some face or figurative thing), turn myself loose to do whatever I think of and sketch as fast as I possibly can. This is somewhere between 5 and 10 'seconds' (I don't allow myself more time than that because then I'll focus too much) and I love doing this. It frees me up from caring too much and they're usually way more fun than fine-tuned drawings.
So, free yo' mind, let loose and make mistakes as fast as you can. You'll be glad you did. Oh yeah, do this a lot and watch the magic unfold.