If eggs are not very fresh, add 1 teaspoon salt and 1 tablespoon distilled vinegar per quart of water (5 mL salt and 15 mL vinegar per L). The vinegar helps coagulate the egg white faster so it keeps a better shape.
Vinegar is not necessary if very fresh eggs are used. Omit in this case because whites will be tougher and not as shiny if cooked with vinegar.