2 oz chilled spirit (gin, aquavit, or Lillet Rouge)
4 oz chilled good quality tonic (like Fever Tree or Q)
Squeeze of lemon or lime
Put your spirit in the freezer ahead of time so it's extra cold, then bury well in a large ice bucket along with the bottles of tonic for serving.
Have your guests assemble their drinks in this order: pour the cold spirit into the goblet, then the tonic (slowly and carefully at an angle to reduce foaming). Slide ice gently into the goblet, then squeeze citrus over top.