Somone mentioned a powerbar wrapper as a tire boot - good fix. But what I carry for a tire boot is better. A simple dollar bill. Actually I carry a five dollar bill. On the occaisional ride where I find myself in need of jingle to spend, I can spend it. And as a tire boot, nothing is tougher.

Mtn Bike
~Small amount of duct tape wrapped around a 5mm allen wrench. Many rides have been saved by this magical product
~Carry a chain tool and a couple of chain links or master links
~A spare derailleur hanger
~ If you don't know what Poison Ivy looks like, learn. Or making that emergency stop in the pucker can turn ugly. I don't get Poison Ivy, so don't follow my lead.

Road
~Don't draft off the back of Hay trailers. Your eyes will take it hard.
~Don't draft off any motorized vehicle and then jump out of the slipstream. Drop back slowly to keep control. I almost ended up in the river once when I forgot that little hint.
~Don't alter your route after telling your wife where you are headed and then call her for help from East Gish wherever 60 miles from home and you are supposed to going out that night. Better yet, just circle around the driveway that day so she can keep an eye on you.

General
~A clean bike is a happy bike
~Over lubing is possible. Learn to do it right.
~WD 40 is not a lubricant. It beats a blank, but not by much.
~If you have a good LBS in town, support it.