Ever since Facebook introduced auto playing videos in the news feed, I've read about how data usage increased along with cell phone bills. This is most evident especially if you're on a plan that allows for less bandwidth on a month to month basis (e.g. 500mb). Simply making a few quick adjustments to restrict videos or other behaviour that requires down/uploading can make a significant difference. I still find it surprising that these settings aren't set by default.
Go under More > App Settings > Video Auto-play
Tap on Profile > Options > Videos (under Settings header)
Google Play Music
Scroll closer towards the bottom under Settings
Go under Camera Upload in Settings
Got any data conserving tips of your own or requests for other apps to tweak? Let me know in the comments below!
Update: Added a few more apps from the feedback I've gotten.