Diet cola has phosphoric acid and caffeine, but most other diet sodas like diet root beer, diet ginger ale, diet orange, etc. do not. So it is only diet colas (Coke, Pepsi, etc.) that are supposed to have the negative effect on calcium. If you don't care about the caffeine, then drinking these other diet sodas would work well.
Here's a link to a study that suggests that: HERE
On the other hand there have been studies that dispute these findings and suggest the only reason routine drinking of soda hurts bone health is because the soda drinkers are drinking less milk. (HERE)
Maybe not the best idea, but I personally drink diet soda every afternoon for the caffeine and fake sugar hit. I've considered replacing with coffee, but haven't been able to yet. Both coffee and tea are supposed to have health benefits (antioxidants), so it would seem that it would be smart to do this eventually.