To expand on a couple tips which have already been provided ...

Ride your own ride.

Especially on organised centuries, it can be very tempting to attempt to keep up with the faster riders. Everyone seems to start fast, no one wants to be left behind, and there's a rush in keeping up with some of the front runners. But this early expenditure of energy can lead to problems later on when you run out of energy.

Instead, start at the pace you usually start your longer rides, and maintain that pace all the way through. Then, if you have a burst of energy toward the end ... go with it.