You will get the maximum amount of juice from citrus fruit when it is at room temperature. Cold fruit can be warmed in the microwave on HIGH for 10 seconds.
Fruit-filled ice cubes look pretty in this drink, and they can be made several days in advance. Place a quarter of a small lemon or orange slice into each section of an ice cube tray. Fill the tray with water and freeze. Store the ice cubes in an airtight plastic freezer bag in the freezer.
Combine the fruit juices and sugar in a nonmetal pitcher and refrigerate. Just before serving, stir in chilled sparkling water.