Definitely carry a notebook/pad with you as often as you can. Ideas strike at the weirdest times. But having a notebook also allows you to write in the brief gaps of time you have while running other errands. For example, I wrote a short story once while sitting in the waiting room at the doctor's office. We're tied to our computers a lot these days, but a pen and paper is portable, wireless and never runs out of battery life. The pages may get filled up, but if they do, then it means it's working.