The most important thing to keep handy is a blank scratch pad: jot down calls you make on-the-fly, questions that come up and you bulldoze past because it's not worth stopping the game for ten minutes, ideas that pop up when the players say or do unexpected things.
I usually end up sending a mid-week e-mail to my group after taking some time to review that. Sometimes all it says is "that thing you did was great--I'd never have thought of that!" Other times, though, it's "hey, I totally got that one wrong; check p.199. It stands, but next time expect it to follow the rule."