The best method to make your allowance go further is to use WiFi whenever you can. That way, you save your data allowance for when you're on the move. O2 has 1000s of free hotspots across the UK where you can use as much O2 Wifi as you want – without paying a penny.
If you are an iPhone user, check whether Apple's 'Wifi Assist' feature is enabled, through your settings.
Go to your mobile data option and scroll to the bottom of the screen. 'Wifi Assist' will connect your SIM to mobile data if the cellular signal is stronger than any WiFi service you are connected to.
If you need more data, you can upgrade your tariff allowance or get a one-off Data Boost, when required.
To add a Data Boost, from your my.plan dashboard, go to:
PRODUCTS > DATA > DATA BOOST > BOLT ON/DATA BOOST > [select user] > ADD