To a martini shaker add 3 lime slices and top with sugar. Muddle until the sugar is completely moistened by the lime juice. If you don't own a muddler, that's OK - You can use the handle of a wooden spoon to mash the sugar and lime.
Add rum, guava nectar, and a couple of ice cubes to the sugar syrup/lime mixture. Close the martini shaker and give it a nice shake for about 15 seconds or so to combine. Pour into a 6 oz glass and garnish with a lime slice if you're feeling all fancy.
Makes 1 drink, which can be easily doubled or triple. Make a pitcher! Ooh - Make a pitcher of this thing and invite me over!