Originally Posted by Emmure

First of all, deactivate Mobile Network on your phone.
And to be 100% sure, check, that no applications are set to auto update, else it's possible, that anytime you manualy turn on mobile networking (to browse facebook or whatsoever) you "waste" data on updates.

settings > Wireless & networks > deactivate Mobile network
settings > Accounts & sync > uncheck Background data and Auto-sync

hope that helped