As Letscook said... just avoid any extreme temperature changes and they should never crack (unless you drop it! LOL!). Fix your soup, stew, roast... whatever while cold. Adding warm meat after browning is OK, but then turn it on and forget it until it's done. Minor seasoning is no problem, but adding cups of cold liquid is! DON'T DO IT! It will crack the crock! It should have been added at the begining.
An alternative to a slow cooker is a Dutch Oven. Adding something cold won't crack a very warm cast iron oven/pot.
"Food is our common ground, a universal experience." - James Beard