Carbonfiberboy 

There was a rage for carbo loading a few years ago, but it's been abandoned. The full deal was that you would really deplete yourself, all the way down to or almost to bonking, just about what you did, and then eat carbs like crazy for several days. The thinking now is that's too much stress on the system and not worth it. What people do now is the last 2-3 days before the event they keep the calories about the same, but cut the fat and protein down, thus the carb percentage goes up quite a bit. So training volume and intensity go down, carbs go up, and you don't gain weight other than from increased glycogen and its accompanying water. This seems to work best. The day before the event, I drink 2 bottles of sports drink slowly, one in the morning, one in the afternoon. That makes sure I'm hydrated, good with electrolytes, and tops up the tank.

A century isn't really a very long ride, though it might seem so now. A good plan is to go with plain water, take Clif bars as described above, and cruise the rest stop tables for what looks really tempting, if there are rest stop tables. I'll often finish a ride like that with extra Clif bars, so I only prepare a couple at a time. One can also use a sports drink, but that's more complicated than just filling your bottles from a spigot and more complication takes more time.

For electrolytes, especially important if it's hot, I use Endurolytes, 1/hr or more depending on the person. I take enough so I'm slightly thirsty and want to drink.

Pay attention most closely to hydration and nutrition for the first 3 hours. Then you got it.