Years ago, I switched to diet tonics and such, but the flavor was never quite right. I'm delighted with the line of mixers from a British company called Fever-Tree, especially their Bitter Lemon mixer. I'm also jazzed by the tonic water from the Brooklyn-based company Q Tonic (its site has an interesting lesson on the history of tonic water, natch). These products were designed to be of the same caliber as the spirits they enhance. You won't find any artificial sweeteners in them, and they tend to be less caloric than their high-fructose corn syrup competitors.