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. Use yolks as soon as they are thawed. Your egg whites will beat to a better volume if you allow them to sit at room temperature for about 20 to 30 minutes.
The Martha Stewart Show, March 2007