On doctor's advice, I have started a fat loss program. The diet change that has made the most difference is eliminating sodas. I love how smooth Diet Dr Pepper is but if I drink one - I won't lose any weight that day and will probably actually gain weight.
Like many people here, I hate to tell others what they should and shouldn't do in their lives. But the evidence is pretty clear (and not just from my personal experience) that sodas should not be a staple of anyone's diet, and should be actively avoided. If you're going to drink one - regard it as a treat. Like a candy bar.
That the soft drink companies are engaging in rent-seeking behavior (like the tobacco industry did) is also a sign that they shouldn't be on the list of allowed purchases.