From Paul Graham: "This is a good plan for life in general: If you have two choices, choose the harder. If you're trying to decide whether to go out running or sit home and watch TV, go running. Probably the reason this trick works so well is that when you have two choices and one is harder, the only reason you're even considering the other is laziness. You know in the back of your mind what's the right thing to do, and this trick merely forces you to acknowledge it."
I would add that the same logic is valid for procrastination: If you can do something today or tomorrow, do it today. Laziness is probably the only thing making tomorrow an option at all.