I find that the nutrition and hydration for a century start the week before, with a gradual build-up toward the big day filled with a balance of carbs and protein. And be sure you're drinking enough for the week going into the ride: you don't want to enter the day in any state of dehydration, even a mild one.
On the day of, my tip is to pace yourself, drink steadily, and make sure your electrolytes stay in check. Eat a well-balanced breakfast (my norm is a bowl of oatmeal with brown sugar and bananas, two eggs, bacon or sausage, whole grain toast, cup of yogurt, a tall glass of OJ, and coffee). Make sure you eat breakfast at least an hour before you ride, to assure that things settle.
On the bike, I tend to use Hammer HEED and Hammer Perpeteum on really long rides. HEED is nice and light tasting, even at full strength, and Perpeteum is a great meal replacement. I also have some energy gels at the ready (love the Honey Stinger Gold), and eat regular food as well (bananas, pretzels, PB&J, watermelon and salted or baked potatoes if an aid station has 'em).
But a lot of it is also training for the bonk, and teaching your body to get past it. My first century rides had me in agony from mile 83 to mile 90, even though I was keeping pace in terms of water, electrolytes, and whatnot. As I rode more long-distance rides, I managed to train my body to work well over 100+ miles. It was a gradual process, but now things are a lot easier because I know how my body reacts over such long distances.
And audition various drinks and supplements until you find one that works for you. Many times, riders find that the energy drink that's good for 50 miles makes 'em want to heave at 80 miles. And that's just a matter of trying out the various options and settling on what works for you.
One other thing: be sure to refuel promptly and properly after the ride is over. That way, you preserve all of the muscular gain and minimize recovery time agony.
Good luck!