Slowly Cut Back
You definitely don't need to go cold turkey, and you don't need to give it up completely. To avoid unpleasant side effects like headaches, over the course of a week, slowly wean yourself down to 12 ounces. Then you can begin to add in decaf coffee, if you choose, or just keep decreasing until you're off coffee completely. If caffeinated soda is more your thing, follow the same plan by slowly decreasing the amount of cola you consume, but don't replace it with caffeine-free soda! Go for herbal tea, seltzer, or water. It'll be a major change, but after a couple weeks, you'll be craving plain water.