Why is everyone all or nothing on this? You just need to get it down to manageable level, not cut it out entirely. Personally, I drink water nearly all of the time, but even I'm willing to get a 4-pack or 8-pack of Jones Soda (mmm.. Berry Lemonade...) every few weeks. I rarely drink Coke or Pepsi anymore though... I swear there is something in those that makes the withdrawal even worse than normal (Maybe it's the HFCS?).
It's really just like drinking alcohol: You need to keep it in moderation. A little caffeine isn't going to hurt you, but if your getting serious headaches after just a few days of not drinking it, you probably needed to start drinking less anyway. Besides, once you stop drinking it all the time, you won't miss it and you'll learn to enjoy it more when you do drink it.
Doctor says ditch the sodas, but to what? I'm sure not gonna switch to just water.
I started mixing in drink mixes into my water to help with it. You can buy all kinds of them at dollar stores or convenience stores, with brands stretching from Hawaiian Punch! to Lipton Iced Tea to Snapple. It really makes you feel like your not drinking water and most of them are only like 5 calories.
After you've done that for a month or two, you can start phasing to just water if you want. At that point you'll be ready for it (or at least I was).