In a smallish Collins glass, muddle lime juice with 1/2 to 1 teaspoon superfine sugar.* Add the few mint leaves, mushing them against the side of the glass. Fill glass 2/3 with cracked ice and pour in the rum.** Pitch in the squeezed-out lime shell and top off with club soda or seltzer. Serve with a stirring rod.
This one responds well to playing around, as long as you keep it within limits. There are some who like to replace the sugar with 2 teaspoons cane syrup; it's hard to find here, but you can make a pretty good substitute by bringing a cup of Demerara sugar ("Sugar in the Raw" works) to a gentle boil with 1/2 cup water; keep refrigerated. In either case, it adds a nice mellowness to the thing. Some -- cocktail historian and restaurant critic William Grimes, for one -- prefer their mojitos to be Draques, sin fizz. That's good, too. We like ours with the fizz, though, but also with a tablespoon of 151-proof Demerara rum floated on top. Another wrinkle has to do with the mint. The Cuban species, "yerba buena," is different from the standard U.S. spearmint; supposedly, the Cuban stuff is available. Worth keeping an eye out for.
* Depending on how sweet you like 'em; we like ours a bit tart.
** Bacardi is, of course, traditional; for a fuller, slightly rummier taste, try Brugal, from the Dominican Republic, or Angostura, from Trinidad.