I'll second what Felix said; I like gels myself, but whatever you go for, make sure you can eat it on the bike. Gels are nice because you can tuck three or four of them in a cycling jersey, and you don't need to screw with coming up with a magic mixture that will last for a day or two in the summer heat beforehand.

Also, make sure you have *plenty* of water. I haven't done a century yet, either (longest ride to date was 83 miles), but a day of running errands will see me sucking my Camelbak (100oz) dry. Bring some gatorade in a bottle if you'd like, but make sure that the majority of your fluid supply is plain h-two-ohh.

I still don't get how some people manage to pull off double-centuries with two bottles on their bike...