What is the shelf life of yogurt?

I occasionally buy tubs of yogurt when cooking more creamy Indian meals, but the only plain yogurt that I can find in nearby grocery stores comes in very large containers, and I generally only use one forth of the tub. The expiration dates on yogurt seem to pass very quickly, but I hate to throw out so much every time I make a dish. How long is it safe to keep yogurt after the expiration date? Does yogurt ever actually go “bad” or does it just lose flavour? How can you tell if it is no longer good to eat or cook with? Is it possible to freeze yogurt? Any personal experience or links to food safety sites with relevant information would be appreciated!