Get a tequila with some personality, and a grapefruit soda that's not too sweet. If you need extra balance, you can always squeeze some fresh grapefruit juice to seal the deal.
.5oz Lime Juice
.25oz Simple Syrup
6oz Grapefruit Soda
Combine all ingredients in a glass and stir. Add ice and garnish with a half-moon grapefruit slice.