Fill a cork bottle halfway up with superfine sugar, the other half with water. Shake vigorously until most of the sugar dissolves, about 1 minute. It will remain cloudy for 5 minutes; after it clears, shake again briefly and it is ready to use. Stored in the refrigerator between uses, simple syrup will last for several weeks.
Adapted from The Craft of the Cocktail, by Dale DeGroff (Random House, 2002).