I always carry an extra tube, 2 co2 cartridges & one of those glueless patch kits. The glueless kits are easy to use, but don't last very long. I've never had to use one, but it's a good backup just in case I have a multiple flat ride. But I'm like a lot of the others, if I have a really bad day, I'll just get a ride the rest of the way.

I suppose if I were going on a longer, self-contained ride, it might be worth it to pack heavier, but, knock on wood, so far I've been lucky.