Use wifi where you can and when at home go into settings, general , mobile and turn off mobile data to force the iPhone to stay on wifi when it goes to sleep.
You can get another 100mb , 500mb or 1gb bolt on as mentioned or if your on a contract phone 02 on 202 to see what's available for you to increase your data.
Remember contract customers are not charged for Going over their data allowance but are throttled instead until your refresh date or you add the bolt on. Take into account any mb gone over in your normal allowance is taken off your added bolt on !
If you take up 4g before the 31/10/13 you can get extra data as an introductory offer ,and still use it on 3G if your not in a 4G area yet.
http://mobi.o2.co.uk/pl/svt/si/mo2couk/po/o2uk/pa/4ghomeIf in payg then it's just a case of adding the bolt on.
