The golden rule in my opinion, which is rarely taken in account, is to monitor the background data use. For example, if you don`t need Whatsapp all day long, just to check some messages from time to time, end its activity and open it, when it`s needed
Most of apps perform their updates, when you connect to a Wifi or use your internet traffic. In this case, you can limit their updating and choose from Setting, when they can connect to internet
