There's one important thing that I've learned over the years of doing marketing both online and off: carry a small notepad and pen or pencil with you at all times. You have no idea how often I've been excited by a marketing idea I've seen, or been inspired with with my own idea, when I'm away from home or the office. Having that little book with me made it simple to write it down rather than lose it in my own memory.
Later I'll be flipping through that little book and land on something I'd written months and months before. Sometimes the idea on that page was exactly the thing I needed at that very moment.