Using a small grater take the zest off of a lemon, lime, orange, or a grapefruit and put it in a large french press. - be careful not to get the white part of the rind, it is bitter. Add vodka and leave over night to give it a chance to absorb the flavor of the fruit zest. The next day fill a glass with ice, add one part of the infused vodka, two parts tonic water (or club soda, I sometimes mix the two to cut down on calories). Garnish with a lime wedge.
The flavor is the strongest if you use the lemon, but it's fun to experiment with other citrus fruits as well.
If you don't have a french press you can strain the vodka instead.