Save the egg yolks when a recipe calls for using only whites, or vice versa. If you don't plan to use the eggs immediately, pour them into an airtight container and freeze. To prevent the yolks from gelling, add a pinch of salt or a heaping teaspoon of sugar for every four yolks. The day before you use the eggs, place the container in the refrigerator, and allow them to thaw overnight.
Martha Stewart Living, April 1999