Always use a clean, grease free bowl as even the slightest hint of grease will prevent the eggs increasing in volume. A metal bowl and a balloon whisk will give the best results. Beat the egg whites slowly at first until they become frothy then gradually increase the speed until the foam is fine enough to hold a shallow peak when the whisk is lifted. If whisked too long they may separate and become grainy and so are more difficult to fold into other mixtures. Always use egg whites quickly once whisked.