Mix a few teaspoons of salt and sugar on a small shallow plate.
Prepare rim, ideally starting with a cold glass so sugar and salt can stick. If glass is not cold, spread some lime juice on rim before dipping cup in salt and sugar. Set aside.
Add all to shaker and mix on ice, thoroughly. Strain and serve immediately.
Sometimes, if I'm being flexible and serving a crowd, I'll skip the shake part, mix all in a large pitcher and chill. Then I serve with ice or a few frozen limes and a splash of soda!