Plan ahead and it will go smoothly. Give your margarita glasses a quick rinse and set them in the freezer for an hour or so, and they will have a beautiful rim of frost on them when you take them out. (If you are making margaritas for 30 or 40 people, you can use small martini glasses, set them on a tray and put the tray in the freezer.) Slice the lime into wedges and cut the center of each with a sharp knife so it rests on the rim of the glass. Have your margarita salt poured into a saucer ready to use.