Most overages come from three apps quietly working in the background.
Turn off background refresh
Keep it for messaging apps only. Social feeds can wait for the evening Wi-Fi.
Move backups to Wi-Fi only
Photo and video sync is the biggest invisible cost on a two-week trip.
Drop video quality
At 480p an hour of streaming costs about 0.3 GB, against 1.5 GB in HD.
syla: switch on alerts at 50% and 80% in the app — you will know before, not after.