Another great tip from Yahoo!:

Less Fat, More Calcium
Low-fat or fat-free yogurt may have more calcium than full-fat versions -- each often contains nonfat milk solids added for thickness. Check labels. Bonus tip: Kale is the one dark-green leafy vegetable you can count on as a source of calcium. Spinach, beet greens and chard all contain oxalates, which bind calcium and make it unavailable for absorption.