How to limit my iPhone 5 data usage?

I had the iphone 4 and for Christmas I bought myself the iphone 5. I have the 2 GB/month data plan with ATT. Before with the four I would use about 1 GB a month but somehow, since the 7 days ago that I bought the 5, I have some how used 70% of my data for the month! I don't know what would have changed, I've been using the phone just like I did the last one. Any tips on what could be happening and ideas for data conservation would be greatly appreciated.

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Data usage warning