To make your own baking powder – some say with fewer metallic undertones than the commercial stuff – mix one part baking soda to one part cornstarch and two parts cream of tartar.
For example: 1/4 teaspoon baking soda + 1/2 teaspoon cream of tartar + 1/4 teaspoon cornstarch = 1 teaspoon homemade baking powder.
[Updated 9/2/10 with this equation for clarity]