When driving through the forest of insanity inside a writer’s mind, it is a good idea to keep the windows rolled up.
Many times I wonder if I’m the only one who sleeps with a notebook next to their bed, or a person who carries around a small notepad at all times. It seems like a redundant idea, but when the inspiration strikes, do you want to be the person who doesn’t have anything to write their idea on? Napkins and receipts work in a pinch, but then they become crumpled or wrinkled in your pocket and then you can’t understand your own writing. Or, heaven forbid, the pen you are using doesn’t work on napkins.
My point in all this? BE PREPARED.
It’ll pay off in the long run. Buy a notebook and pens. Keep them (at the minimum) in your car or buy one small enough to fit in the pocket.