I believe in patches! Tubes are inexpensive, but can add up fast. I'll also follow suit in recommending kevlar belted tires. It isn't perfect (something put a big cut in the unprotected sidewall of my rear tire on my recent tour), but I've found shards of glass embedded in the tread of my tire that stopped at the kevlar belt. Another thing to keep in mind is that kevlar doesn't do a thing to prevent pinch flats, so keep your pressure up and mind the road! On the same tour that put a slash in one of my tires, I managed to blow both tubes at once when I hit a nasty hole at speed.

Anyway, conventional wisdom is to carry at least one spare tube for a quick change on the road, but to patch the punctured tube later. Unfortunately, really big holes, like from a tire blow-out may not patch, forcing you to replace the tube.