Pick up the official trailbook before you leave. (http://www.atatrail.org/trailbook.cfm) It tells you where the towns and campgrounds are, has ads from local restaurants, and (perhaps most importantly) explains how to find mile marker zero in DC.

Another suggestion: replacement spokes. I rode the trail on a hybrid bike without any trouble, but one night at a campsite on the C&O I ran into a couple on road bikes. Two of his spokes had snapped, and they had a horrid time finding replacements. I carry a kevlar spoke just in case (http://www.sheldonbrown.com/harris/t...ing.html#spoke). I've never had to use it <knocks wood> so I can't give you a review based on personal experience -- but I've heard good things about them.