Some more advice from experience:
I progressively gave up caffeine (which I took in in the form of Pepsi) over the course of a year. I cut back from three to two to one can a day, but this was originally at the dentist's urging. Once I was down to one, I decided "Why not cut caffeine out altogether?" My grandfather has a slight heart flutter which I seem to have inherited, and my mom has hypoglycemia, so this seemed like the right thing to do, seeing as caffeine is very bad for both these conditions.
About the time I went about dropping out my daily can of Pepsi, I started to get very tired; I could hardly stay awake in school. I didn't associate this with caffeine withdrawl at first, and my doctor diagnosed it as mono. Blood tests ruled this out, however, and hypoglycemia was ruled out as a cause, too (although drinking fruit juice in order to get more sugar seemed to help). I later realized it was because of the lack of caffeine in my diet; my body was not getting the usual boost it was used to.
As a Pepsi replacement, I usually use white grape juice (the sugar helps a lot), sweetened iced tea (again the sugar, and the small amount of caffeine gives me a small boost), and Sprite. I think that Sprite is possibly the best choice for cola addicts, since it is caffeine-free and tastes similar to cola; caffeine-free colas just don't taste right. Iced tea might be good for coffee addicts, I'm not sure.
Occasionally, I will slip to the dark side and have a glass of Pepsi. This is a bad thing. My heart starts racing, I get anxious, my hands shake, and I'm often tired the next day. Don't ever do this if you need to do something physical after! You will be sweaty and short of breath.
Hope my advice can help someone else out there kick the caffeine habit - do it, you'll reap great benefits later! (Being able to get up in the morning without a caffeine boost is just one of them!)