It's amazing how much data some apps consume nowadays. If you're on a wi-fi network, that's not a big deal. Your mobile contract (or PAYG allowance) may not have much mobile data bundled, however, so you could rapidly find yourself racking up charges just for a few extra measly MB of downloads. Fortunately, Apple's iOS operating system now reveals how much mobile (aka cellular) data that each app consumes. Simply go to Settings, Mobile (or Cellular), scroll down and you can see how much mobile data your various apps have consumed. You can then decide to stop certain apps from using mobile data. Easy!
Another way to cut data usage is to turn on Do not Disturb (also found in Settings), in Scheduled mode. Simply define the time period that you want the phone to be effectively off-line, and this should cut down on data usage due to push notifications.
One last trick for avoiding nasty surprises on your mobile phone bill is to turn off MMS messaging. MMS messages (ie, texts with pictures) are almost always charged extra by your network provider, so just turn off the capability inside Settings - Messages. Amazingly, an SMS with an emoticon (ie, a smiley face or something) will be converted to an MMS, so you may not even be aware that you're sending MMS messages!