Meringue is temperamental. Getting it right can be a tricky process, but these tips can make a difference:
1. Egg whites are easy to separate when cold; then let them reach room temperature before beating. Whites must be clean, with no yolk or fat attached.
2. Copper's acidity stabilizes egg whites, so use a deep copper bowl with a rounded bottom. Or beat in a glass or stainless-steel bowl (never a wood, aluminum, or plastic one), and add a pinch of cream of tartar for every 2 egg whites just as the whites get frothy.
3. Superfine sugar dissolves quickest and should be added a tablespoon at a time when meringue reaches a soft peak stage. Never add salt; it decreases stability and increases beating time.
4. Overbeaten whites will collapse. To revive them, beat 1 egg white until frothy, then gently fold into overbeaten whites until they're shiny and moist again.
5. Humidity and meringue don't mix, so add 1 tsp. cornstarch to the sugar on humid days.
6. Bake meringue at 350° for at least 15 minutes to eliminate any risk of salmonella.