I know DealDude posted a thread about setting hard limits on your mobile phone, but another option if you're using an Android phone and with Virgin Australia is the app that looks at usage:
The Virgin Usage Meter is a simple app that displays your data and call usage against your billing cycle in an easy to understand graph. It has multiple account support so you can keep an eye on other family members. It also includes a convenient widget to ensure your usage is always at your fingertips.Icon was designed by Kat McGowan who is an awesomely talented designer!
This app uses data from the Virgin Members website and as such is affected by any alterations Virgin Mobile makes to this website. I will try to release fixes as soon as possible but in some cases the time required for fixes is extended.
This app is not a substitute for common sense. Always check your monthly statements and regularly check the updated timestamp on the app to ensure you're viewing the latest information. If you're installing the app on behalf of somebody please ensure are aware of the above information.
This app has no affiliation with Virgin Mobile. Virgin is not responsible for any damages be they monetary, software based, or physical caused by the usage of this app.
This app comes with no implied warranty, while I do my best to provide an app that is safe and error free you use this app at your own risk. I will do my best to support you via email but I will not be held responsible for any damages caused due to the usage of this app.