When beating egg whites, be sure the bowl is spotless and grease-free; metal bowls seem to work better than plastic. Even a tiny speck of fat or egg yolk can cause whipped egg whites to have less volume.
One baking step that CANNOT be done ahead of time is beating egg whites. After 5 minutes they will lose volume. Egg whites will whip better when at room temperature because the protein in the eggs is more elastic and will whip up into more of the tiny air bubbles. If the eggs are colder, they will take longer beat. Unfortunantly, in high humidity, you will sometimes not be able to get the correct volume no matter what.