Plain soda water or talking rain(carbonated water with a slight fruit flavor - nothing else). This helped me curb my Coke habit. Something like 70¢ a 2-liter and I get my soda fizz fix without the calories or any of that stuff that is in diet sodas. Pretty good with lime as well.

A change to a healthy diet is a bit of a lifestyle change... it's a good thing. Good luck!