I've seen a lot of girls at gatherings and parties that I've been to mix their vodka with Crystal Lite. And I have to admit I've tried it too and it's pretty tasty. Usually Crystal Lite is less than 10 calories per serving. You just have to mix it up like you would kool-aid and then throw in your vodka and voila!
Flavored sugar free seltzer water comes in tons of flavors, raspberry, kiwi strawberry, peach mango, mixed berry ect... every grocery store should have it. Coke zero and a lime wedge go great with light rum for a low calorie drink.