I like the post about books. You shouldn't bother playing a multi until you have read at least one book on the subject. Or at least you shouldn't play a live multi.
As for coffee....I might have a biased view on it, since I don't drink it often. Coffee and energy drinks give you a boost, but they also lead to a crash. For that reason, I avoid them when I play a multi or sit for a 12 hour session at a cash table. Unless you keep drinking them constantly, you will slow down later and lose focus and concentration. In a multi, this is suicide. You spend all that time and energy making it to the final stages, you might even get in the money, and then you crash and you misread a guy or move in before you really have to and you get popped. That's never happened to me, but I see it in EVERY multi I have ever played in. You see a guy fired up and played pretty solid decent poker all day, and then after the dinner break, he just slumps down in his chair and calls his whole stack on some hand he never would have called with 4 hours earlier.
Of course that's a stamina issue, but the coffee and energy drinks rob you of whatever staying power you might have. I say save them for sit and goes and short sessions late at night when you need a concentration boost.
I think its important to be well rested. Get a good long sleep. If not, don't play in the tourney. Also, eat small amounts of high energy food, like an energy bar or some fruit. Eat a light dinner...too much food in the middle of a multi will rob your brain of oxygen rich blood that your body will use to digest.
Stay hydrated. Some players never even take a sip of water. Your concentration lags when you run low on fluids. And you won't even notice because you're just sitting all day. Drink water or fruit juice.
Keep your mind off of distractions. Think about your life's problems after you bust out or, better yet, after you win the thing.