This drink is currently so popular that Bacardi gave it it's own website. I make about a quart of mix at a time (I have a jug that seals to hold in the carbonation). It is shockingly smooth and addictive, the combination of lime and mint is amazing.
FYI, I buy superfine sugar in what is basically a milk carton. Superfine sugar has finer crystals than regular and dissolves very smoothly.
1 part Bacardi Rum
3 parts Club Soda
12 Mint Leaves
1/2 part Sugar
Place 12 mint leaves, 1/2 part sugar and 1/2 lime in a glass. Muddle well with a pestle. Add Bacardi, top off with club soda, stir well, and garnish with sprigs of mint or a lime wheel.