On most Android phones under 'Data Usage' You'll see a setting for restricting mobile data. You can set a cap on the data usage for a given time period, generally by using a slider on a graphic chart. I set it from one billing period to the next. You can also set a data warning to alert you as you approach that cap in a similar way.
Under that setting there is a list of apps that are frequently used. If you want to 'go nuclear' on the limits you can tick the 'Restrict mobile data' on some or all of them. This will a) prevent them being used on mobile networks, defaulting to Wifi only use and b) prevent them from downloading background data over mobile networks, which can be a significant part of your usage if you're not aware of it. Careful selection of which apps to limit can give you dramatic data savings (I went from exceeding the limit to 2mb/month give or take)
I'm not sure of the deal on iPhones but it's not a huge leap of the imagination to assume there is something similar available, probably in a similar part of the settings.