I do 3 or 4 centuries a year, usually over the summer on long weekends. I don't really prep for them since I do a normal commute of 28 miles/day, which makes 140 miles/week.
All my centuries are unsupported; I usually decide a day or two in advance that I'm going to do one the upcoming weekend. When I ride a century I usually carry all the same gear on the bike I do when I'm commuting, but instead of clothes and a towel, I'll carry extra hydration (cherry kool-aid, water, and usually an energy drink or two), some food (I usually take bananas and a couple of PB&J sandwiches). I make sure my cell phone is fully charged and let my wife know the basic route I'll be taking.
I'm generally not super fast, because for me it's more about enjoying the ride and seeing the scenery, but I've never failed to finish. The easy part is the second 50 miles, since you've already ridden out and *have* to ride the rest to get back home
. It is on those rides that I wish I didn't live on top of a hill, though.