Cut the lime, lemon and orange in half, then cut each half into four slices. Place two slices of each in the bottom of a chilled glass. Add sugar and fresh mint leaves. Pulverize with a muddle. If you don't have a muddle, use a heavy spoon handle. Mash the fruit and the leaves to release the juices and the flavorful oil from the rind. Fill the glass with ice and add the light rum. Stir well and garnish with a fresh sprig of mint.
If you prefer your cocktails a little more mellow, double the fruit and the sugar.