Have a sharpened pencil and stack of paper handy always. I can't say how many times it's saved me a lot of trouble programming some behavior because I was able to draw a picture of what I wanted it to do.
I am opposite of that. I can't stand paper and pen/pencils, so whenever I need to visualize something it's Paint time. And if I need to note something down it's Notepad time (well, Notepad++ lately).
