I find that being insane helps considerably. Look at how many great artists, musicians, and authors were not quite right in the head!
Having a short attention span helps as you may notice things that others don’t, and you may find them interesting enough to actually think about for a few moments (or longer) in order to relieve boredom. ADD/ADHD also counts as “insanity” for these purposes.
I think that @iamthemob has it right in that planning something makes it more of a chore/job than a truly creative effort. You can’t force it; you just have to flow. The most creative ideas I’ve had just popped into my head randomly as I had my mind elsewhere, or had them triggered by something that I saw as I was meandering about aimlessly. (Yes, @Cruiser, inspiration is all around us!)