[LucyRene]

I did not get an unlimited data plan. Any tips on saving data usage? Should I leave it on WiFi all the time?

[GSDer]

You could certainly leave both mobile data & WiFi turned on all the time - as long as you have a good WiFi connection your phone will use that; if you get out of range of the WiFi connection then your phone will automatically switch to using mobile data.
However, to save the maximum amount of your data plan, I'd recommend leaving mobile data turned OFF, and only turn it on when you need something then turn it off again.

Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 2

[Uncle Reemus]

You can set a mobile data limit in settings if you need to. It will let you set a warning and also a data limit that will turn off mobile data and only allow WiFi.

Sent from my AT&T Galaxy S3

[justpastfinish]

Download onavo from the market and let it do its thing for a few days. It will tell you what apps use all your data

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[meyerweb]

Go to Settings / Data Usage and scroll down below the graph. by tapping on each of the apps displayed, and checking the "restrict background data" option, that app will no longer sync data in the background when on a cell connection. It will continue to do background syncing on WiFi. Do this for apps that you don't really need immediate updates for.