Like Pete, many of the things sstorkel mentions are things I'd rather limp with to the next bike shop. Wheel and chain stuff you should be able to fix on the road, and a lot of things can be prevented with a bit of attention.

Spare tube? Know how to find what caused the flat and fix it? Absolutely! But pump the tires up every day or two, and that'll help prevent snakebite flats.

Spare tire? I've used one, but only once. Maybe take a light, foldable tire.

Chain trouble? Lube the chain once a week, wipe it down every day or two. Borrow a ruler to measure the chain when you lube it. Small chain tool and spare master link may keep you moving.

Start with a new chain, new tires, and new brake pads (Kool Stop Salmon pads). Keep a couple of 800 numbers for tire suppliers on hand in case you need to have a spare overnighted a day or two ahead for those times when no nearby bike shop has anything over 700x23. When the chain starts to wear, ride to the next bike shop and have it replaced. Take spare brake pads and have them installed first chance you get when they wear past the knobs.

It's a good idea to have a mechanic show you how to trim the brakes and derailers for when the cables stretch. Adjust the brake cables as necessary and leave the derailers alone.

Take a small bag of extra bolts, washers, and nuts in common metric sizes. When you do your weekly chain lube, check all the rack bolts for tightness (and replace anything that fell off immediately!).