To replace 1 teaspoon baking powder, I use 1/2 teaspoon cream of tartar plus 1/4 teaspoon baking soda plus 1/4 teaspoon cornstarch. I prefer this mix because the latter ingredient keeps the mixture from clumping together, and I think it provides better lift.

No matter the exact ratio you choose, you'll get better results if you use an acidic wet ingredient in your batter or dough. The acid helps neutralize the alkaline nature of baking powder; without it, baked goods sometimes taste unpleasant. Buttermilk is an obvious choice (which can directly replace milk in a recipe), or you can add 1 T lemon juice per one cup of milk.

If you have cream of tartar and baking soda you can make your own:
2 tablespoons cream of tartar + 1 tablespoon baking soda (according to Edna Lewis' proportions)
OR
1/4 teaspoon. baking soda + 5/8 teaspoon cream of tartar (according to Joy of Cooking's proportions)