Short answer: Nothing. We’ve found that stones do not last forever, though Jeff’s first one lasted through 11 years of daily baking. It seems clear that the 1/2-inch thick ones are very durable, and and some brands (not all) of the 1/4-inch thick stones are less so.
Incidentally, you can often use the broken pieces of the stone, depending on the size and shape of the fragments.
Alternatives to these ceramic stones include the newer cast-iron pizza “stones,” or even just a cast-iron skillet, which we’ve found work quite nicely. Or try a Dutch Oven http://www.artisanbreadinfive.com/?p=552 or even the baking cloches http://www.artisanbreadinfive.com/?p=566, though our best guess is that cloches, being ceramic, are eventually going to break like the stones. Iron, definitely not–these should never have a problem with cracking.