I have the same vice. I don't drink coffee so coke is my caffeine for the day. I haven't cut it out completely yet (due to the headaches, etc..) but I have cutback significantly.
1. About every other week or so, I let myself have less. I started by going from a bottle (the little ones), to a can, now I am on fountain soda which has a ton of water. Since I can control how much of it to put into the cup, I am down to about half a cup at this point.
2. If I am in need of something fizzy I go for a sprite. It still isn't good for you, but won't contribute to the caffeine habit. Even with sprite, I'll only take a little bit to get the fizz.
3. I'm not a big fan of water, I literally have to force myself to drink it. Now that I don't drink much soda, I'm finding I add Vitamin Water Zero in with my water intake. I am someone who constantly needs to have a drink nearby, so if I have water and Vitamin Water around me, it keeps me away from the really bad stuff.