Ah, buttermilk--that magical ingredient that gives so many delicious baked goods their characteristic lightness. Unfortunately, it's not exactly a staple in everyone's household. What's a girl to do when the family is clamoring for buttermilk pancakes and all she's got is plain milk? Well, she could just adjust the recipe and use the plain milk (for the record, you simply use the same amount of milk as buttermilk called for, eliminate the baking soda and double the baking powder to do this), but the flavor and texture of the pancakes is just not going to rival that of real buttermilk ones.
Fortunately, there is a remedy! You can make your own buttermilk substitute, called "sour milk." For every one cup buttermilk called for, measure out a scant cup of regular milk, and add to it one tablespoon of lemon juice or distilled white vinegar. The lemon juice will give a more authentic flavor, but either one works. Then, stir your concoction and let it sit for five minutes, and you've got sour milk--but in this instance, that's a good thing!
I use this trick all the time and very, very rarely do I buy buttermilk. One reason is that buttermilk is quite a bit more expensive. Secondly, in my area, it is sold only in quarts, and I rarely need exactly one quart of buttermilk, and I hate to waste. Thirdly, in my area, buttermilk is only sold "lowfat" (1% milkfat), and I find that most baked goods really taste best when made with full-fat dairy products. By using "sour milk" in place of buttermilk, I can ensure that I have full-fat ingredients.
So there's the tip for today. Even if you still want to stick with purchasing buttermilk for the most part, I hope this can help you out in a pinch!