One of the things I like about running a game is the demands it puts on me. Most of the time, I don't have to be at peak operational capacity. It's nice if I am all there, but when it comes to hanging out, school, and most of my other hobbies, I can get away with running at 50% or 60%. Gaming doesn't give me that luxury: I have to be 100% there as much as possible. It takes pretty close to my full mental capacity to simultaneously visualize the game environment, track player interest and involvement, easily call up important backstory details, remember where all my different notes are, roleplay NPCs properly, and think up interesting responses to player actions.
Accordingly, I've got a lot of little things that I do to get myself into the right state of mind and stay there. In no particular order:
Shower: And all my other hygiene routines. If I'm not clean, I get uncomfortable. If I'm uncomfortable, I can't run the game.
Power Shirt: At the very least my clothes have to be clean. But it helps me to get into "game mood" if I'm wearing the right clothes: jeans and t-shirt, usually black, generally geek related. My batsignal shirt is best, but I've got a bunch of other ThinkGeek type shirts that will do the trick.
Food: Need to have eaten some food. Need to have eaten the right food. If I haven't eaten, I get grouchy. If I've had too much sugar or caffeine or junk food, I either get way overstimulated or I go to sleep. I generally stick to baby carrots and cherries at the game table. Tasty, crunchy, and won't make me crazy.
Water: At the table, I drink a lot of water. Ludicrous amounts of water. I'm drinking water constantly. It keeps my voice from going out and keeps me from getting dehydration headaches. Best of all, it gives me a great excuse to leave the room and take a five minute break to think. (My players get all nervous when I do that, too.)
Books: It helps to have a big ol' stack of game books nearby, even if I'm not going to use them. I did this unconsciously for a while: when I was running a d20 system, I'd just cart out all my d20 stuff out of habit, even if I was running Arcana Evolved and so my d20 Modern books and so on weren't going to be much help. (I've got about two feet worth of books for various d20 system.) These days, I like to have my AD&D Dungeon Master's Guide close at hand, even when I'm running Traveller, so I can tap the Awesome Power contained therein. (NPC charts, mostly.) And I brought my copies of Fight On! to the last game, for pretty much the same reason.