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.