You can save data usage by turning off some of your apps, like the location, that always runs in the background. You can turn them on when you need them, ie for "maps" when you travel. You don't need location in your day to day use.
You should be able to tell your phone not to update apps without asking, or to only update apps when using wifi, so it doesn't do this automaticallly using up your data. Otherwise all of your apps will update themselves constantly in the background using your phone data. Turn off any automatic syncing that you don't need also. Turn it on when you have wifi somewhere, except for your email, if you want to get it everywhere you go.
Cell phone and simple texting work without data.
If you want to download some larger items, like movies, audiobooks, etc. do it using wifi, when you have it, not your phone data.
Protect your phone data and it will last a long time.
