whey), add some lemon, and refrigerate for two hours. See here for detailed recipe.
This tutorial mentions the use of a fine mesh kitchen sieve to drain the yogurt, but cheese cloth or a strong paper towel over a colander works well too. I use the colander/paper towel combination over a large bowl to drain the yogurt. Instead of discarding the drainage(whey), save it for baking and cooking. I use whey in homemade bread.
For something extra, save a portion of the drained yogurt; skip the lemon, drain overnight, and you'll have creamy cheese!
Image for wikipedia.