This is an interesting article from BikeRadar.com about an independent bicycle tire testing lab and what they do there. Much of this applies to weight weenies, but there is some decent stuff for us ordinary folks as well, such as:
Puncture-resistant belts work but they’re not created equal: Nylon, aramid and other belts placed under the tread do help ward off flats but there are benefits and trade-offs to the various materials. Tougher ones like aramid are durable and highly cut- and puncture-resistant but their stiff nature sucks up a lot of energy, contributing to rolling resistance. More flexible ones like nylon aren’t as bulletproof but offer a better compromise if you still want to retain good performance.
Well, that may be, but I started using the aramid (Kevlar) belts in my tires a couple of years ago and haven’t had a flat since where I was having at least a flat a month before. And since one puncture can ruin my whole day, I think I would rather put my effort into pedaling rather than changing a tube.