1. iPhones are coming with up to 64GB of storage now, use it! stop streaming pandora and netflix, store your content locally
2. Use the Opera browser. Data is compressed at the server level by Opera, reducing your usage.
3. Block ads. They are not only annoying, but they steal your precious bandwidth. I personally VPN to my home, and use my own DNS server which filters internet ad servers.. But that is for another post. There are adblock apps for Jailbroken devices.
4. Jailbreak and install SBS settings. Toggles are available to quickly turn your mobile data on and off, without digging through the settings app. Things happen in the background and transmit data, so if you don't need it, simply turn it off.
Using a combination of these tips got my data usage from 700mb-1200mb/month down to under 100. The only thing I feel like missing out on is a bigger bill from AT&T.