I used to drink Pepsi every day, then it turned into several per day. When I started to hit like 5-6 a day I realized it's time to slow down. I was able to bring myself to 1 per day. Then later on every couple days. Now, I don't buy it anymore, and only time I have one is if eating out, or maybe when I visit my parents. Basically it's more of a treat than a daily thing.
The trick though is trying to find something to replace it with other than water. Yeah you should drink lot of water too, but sometimes you want something more. Like if watching a movie or something. I kinda inadvertently replaced it with coffee myself. I slowed down drinking pop but now drink more coffee. I don't put anything in my coffee and drink it black, and there's more studies saying coffee is ok for you than those saying it's bad, so I'm not too worried. Another thing I was drinking for a while too is Club Soda, but it's harder to find. Not all grocery stores have it, and the ones that do don't always have it in stock. Pop on the other hand you can find everywhere. I think that's part of the issue in general with nutrition, all the unhealthy stuff is constantly being put in front of us and not the healthy stuff. So it really takes a lot of effort to avoid it and seek the healthy stuff.
Oh as a side note, if you're going to have a pop, you may as well get the real thing. Diet is actually much worse for you.