I just want to point out that using diet drinks as mixers increases the speed of alcohol absorption. You should be able to get drunker faster from booze and diet (fill in the blank) then with the same amount of gin and tonic (diet tonic exists). Thus drinking less in the long run and consuming fewer calories.

I knew a very heavy drinker (who was also a rather large guy) several years ago who actually lost a visible amount of weight from switching his beverage of choice from rum and coke classic to rum and diet coke.

There are tons of articles about this, here's one.