Re: Android 4.4 kitkat S5
You'd have to restrict background data and disable sync while on mobile data. Android is always doing something in the background, but the OS alone will not take more than a couple of Megabytes out of your data plan a month. Other apps, however, can take a toll, like e-mails coming through, apps with notifications like Whatsapp, Facebook, Twitter, etc., and you have to be careful with self-updating widgets, like weather ones, that usually download new data every hour.
You can also set Android to cut off your mobile data access if you ever reach a certain limit to avoid overages. And there are also compression tools that can help you reduce your data consumption. Chrome has such a feature available, as well as Opera. An app called Onavo can also help and this one claims it can compress all incoming data, not only web pages.