Welcome to Android Central! If you don't want to turn off your Jetpack, you could turn off your phone's auto-sync and only turn it on when you want it to sync. If your phone is set to auto-sync, then all Google services will sync at relatively frequent intervals, which can contribute to data usage. You can also check all of your apps that refresh data on the web (like Facebook or weather apps), and change their refresh rates to the longest interval possible, or manual. For the Google Play Store, make sure it isn't auto-updating apps--and if it is, there is a selection to do it on wi-fi only.
There's also a setting in Settings>Data Usage if you press the menu button, for "Restrict background data." This will prevent certain apps from syncing, but the tricky thing is that it doesn't really specify what is prevented.