You can download an app which will warn you when you're approaching and over your limit, I find it very useful - 3G Watchdog is the name for an android one. I don't know about Apple but would imagine that there is one.
250Mb isn't that low. I had a 100Mb limit at one point and it was my only internet access, I kept on top of it and it was doable. So, tips:
Make sure you are automatically connecting to your home wifi and anywhere else you go which has wifi, work, etc.
Go into settings, both the phone settings and the settings for every individual app and disable automatic updates (or allow only on wifi if you like updates), reduce the frequency at which apps like facebook, twitter, email, etc sync, as this all uses data.
Avoid using picture-heavy apps like instagram on mobile data, and you can also change a setting in your mobile browser to disable loading of pictures if you mostly browse text-heavy sites like mumsnet (it has the bonus of avoiding most ads, too).
Don't ever upload photos to facebook, whatsapp, etc, unless you're on wifi. Don't use streaming apps like music, videos, etc unless you are on wifi.
If you got your contract in 2012, though, you're well overdue an overhaul. Never just let the contract roll over after the 2 years are up, that is the worst thing you can do, always go to them and see what deals they will offer you.