I understand what you mean with the diet coke habit, although for me it's just plain old coke-classic. Water? What's that?
I'm slowly giving up coke, and it's been a damned hard struggle for me. It's just like any other comfort food, plus the added addiction to caffeine. I crave it, like a thirsty man craves water.
I'm not sure quite how I got into the habit of drinking water this past month. I have a water bottle (that is, a big one) in my kitchen, and it just became easier and easier every day to just fill up a glass or two from it. Now I fill up a bottle and drink it at the computer.
After trying to quit caffeine cold turkey, I'm now just limiting myself. I was so damned irritable! So, now I just have one can of coke a day, and drink it very slowly. We'll see how long I can last. I'm hoping to cut down to every other day in a couple of weeks.
Good luck, and if you need anything, or just someone to talk to who's going through exactly the same thing, drop me a line: firstname.lastname@example.org