Just think sad thoughts. Like about your puppy dying. Or something. Unless you hate your puppy, then you should think of something else that is serious.
Just avoid finding yourself in the situation i'm usually in- don't let people see you making the faces you're trying to draw. I have been asked so many times if i'm alright.... (and that's the times when people have felt safe enough to approach, I think)
Anyways, seriousness is overrated. Everyone does serious. It takes real balls to step outside of that and be whimsical. :3