Save the slightly older eggs for boiling—they peel better than the fresh ones.
Make sure the eggs are at room temperature too (important for good aeration). It’s harder to separate warm eggs, so you may want to start with cold eggs out of the fridge, separate them, and then let the whites warm up.
Separate the eggs very carefully
Don’t use the white if it contains any trace of yolk (save those for scrambled eggs). Use three bowls to eliminate the risk of losing all your eggs with one slip-up: one for separating, one for the whites, and one for the yolks.
If the egg whites are cold, let them sit at room temperature for about 30 minutes or put the bowl in a bath of lukewarm water to warm them up faster.
Use a deep mixing bowl and very, very clean utensils
Even the smallest bit of oil can keep the eggs from expanding. A copper bowl is ideal for beating eggs (but other bowls will do fine, too). If you’re feeling strong, use a whisk to beat them by hand. An electric hand-held mixer or a stand mixer with a whisk attachment speeds up the process.
A little acid adds volume and stability
Once the eggs foam and start to puff up into the earliest stages of soft peaks, you can sprinkle in a bit of cream of tartar, a few drops of vinegar, or lemon juice. Keep beating the eggs continuously. (Stopping to take photos, as I learned, can cause the mixture to become unstable.)
You'll know you're at the soft peak stage when you pull the whisk out of the mixture and the whites form peaks with the tips flopping over. For sweet preparations, this is when you gradually add in sugar, a tablespoon or two at a time. Superfine sugar is ideal because it dissolves easily. Adding sugar helps to stabilize the eggs, which means less risk of weeping egg whites.
But be careful not to add sugar too quickly or too soon—you'll risk losing volume.
You've reached stiff peak stage when the egg whites are smooth, moist, shiny, and the tips can stand straight up. This is what they should look like just before they are folded in with other ingredients.
The earliest signs of overbeating are little granules on the side of the bowl (as noted in this photo) and decreased volume. After that, the whole mixture looks dry and curdled.
If it's really overbeaten, the structure of the egg whites will break and liquid will weep out. Try beating in another egg white to help recover the mix.