I’m with GiffGaff and have gone for the cheapie £10 package which limits me to 1Gb of data per month. Regularly I was hitting 700-900Mb per month, which is fine – but I noticed I was gradually using more. No real reason other than I was out and about and getting more and more reliant on apps, etc.
A simple change to my settings saved me a huge amount of data and I’m now hitting around 300Mb per month. Simply ensure that your app updates are set to run automatically only when you’re on wi-fi.
Aside from things like streaming media, this is probably one of the biggest data downloads you do on a regular basis. There’s usually very little need to have an app update *right now* (and if there is, you can force an update manually very easily), so just leave them till the next time you’re in the house when they should kick in automatically.