This is a pretty basic tip, but perhaps a handy one for novice GMs: Your players will forget stuff, so have spare dice, pencils, pads and other supplies on hand. This goes double if you’re running a convention game.
My RPG to-go box includes three full poly sets (d4 through d%), two full sets of 10d10 (for White Wolf games) and enough d6s to choke a horse, plus two pens, two pencils and two clicky erasers. When I GM, I also have notepads on hand, as well as lots more of all of the above.
One of the other GMs in my group, Don, used to keep a spare set of dice on hand called the Dice of Shame. The Dice of Shame were the ugliest, most mismatched dice his group could find, and if you forgot your dice, that’s what you used. I’ve always gotten a kick out of that idea — keeping spare stuff for your players doesn’t have to be boring.