Equipment: Because any residue of fat on beaters or bowl will prevent egg whites from beating properly, they should be spotlessly clean. The bowl should be stainless steel or glass. Plastic bowls can retain a film of grease.
Egg temperature: It's easiest to separate eggs cleanly when they are refrigerator cold. However egg whites whip to greater volume when they've had a chance to warm a bit, 20 to 30 minutes. Always start by separating the eggs first. Let the whites stand at room temperature while you prepare the dish, sauce and other ingredients.
Cream of tartar: The air beaten into egg whites can be lost quite easily. A small amount of acidic ingredient, such as cream of tartar, acts as a stabilizing agent. A bit of lemon juice or vinegar will also work.
Salt decreases egg-white foam stability, so it should be added to the other ingredients.
Beat egg whites just until they are stiff peaks but not dry and no longer slip when the bowl is tilted. When underbeaten, whites will not achieve full volume. Overbeaten whites form clumps of dry puffs that don't hold air well, are difficult to incorporate when folding and do not expand properly when heated.
Gentle folding is the key to maintaining volume: Combining heavier mixtures with beaten egg whites can knock the air out of them. Begin by pouring the egg yolk mixture over the beaten whites, not vice versa. Then gradually and gently combine the mixtures by folding, rather than stirring. Using a rubber spatula, start with a downward stroke into the bowl, continue across the bottom, up the side and over the top of the mixture. Come up through the center every few strokes and rotate the bowl often as you fold. Fold just until the color of the mixture is uniform, with no streaks of white remaining.