Nine years ago I stopped drinking soda because I was pregnant with my first child. And, in the past nine years I haven't gone back. Soda is terribly expensive especally if you buy name brand. If you want to save a big chunk of money, cut back or elimate soda all together.
If the thought of quitting cold-turkey sounds too difficult, try out by reducing your soda intake by 50% at first. One you hit that milestone, cut back to 1-2 per week, then cut back to only drinking soda at social events.
Instead of soda, I drink water or water flavored with a drink packet. My savings are terrific. (Now, if only I could get my husband to follow this advice.)