I'm assuming you have the Sony Xperia Z?
Disable automatic application updates. You should be safe then.
Also, the phone has a mobile data limiter. Set it to warn you when you reach a certain amount of data used, and set it to cut off when you reach another amount of data used. Personally, because I'm on Cell C and get 100mb a month free, I set the warning to about 55megs and cut it off at 85megs.
I noticed on Whatsapp that I dont get messages on time when the phone goes into sleep mode, but I haven't figured out yet whether its because of the crap Cell C service or the phone itself.
Also, there is a problem with getting notifications when you enable stamina mode and the phone goes into sleep mode, even though you have the app on the whitelist.