Have a riding buddy. The most important tip I can offer is not to go it alone. My first century, I went out of state and cycled unfamiliar roads without any support . Within the first few miles it became clear to me, that was not a good idea.
Although it can be hard to find someone to cycle a century with you, the benefits of finding a riding buddy make it worthwhile. The most important benefit is that it is safer to have a riding buddy if something unexpected happens. Barring any unforeseen problems, a riding buddy can offer valuable encouragement to help keep you going.
Finally, the shared memories of conquering what initially may have appeared to be an impossible goal of cycling 100 miles in one day, can strengthen your friendship. And, it’s nice to have someone to celebrate with that understands your achievement.