Download Onavo Data Usage which shrinks date, don't have location services turned on unless you need them, don't use push notifications and I believe doing the following saves data usage:
To change your device’s reporting options:
Connect your iPod touch, iPhone, or iPad to your computer.
In iTunes, hold down the Control key on your keyboard while you click your device (below Devices).
Choose Reset Warnings from the menu that appears.
The next time iTunes asks you for permission to send information, follow step 2 in the above section to set your reporting options.
Reports are submitted to Apple using your Internet connection. Your computer and device keep the information for one month after a report is generated, and then delete the information.
(section 2 referred to is:
Do one of the following:
To allow iTunes to send information automatically, select “Do not ask me again,” and then click Agree.
If you want iTunes to ask for your permission each time it sends information to Apple, deselect “Do not ask me again,” and then click Agree.
If you don’t want your device information sent to Apple, select “Do not ask me again,” and then click Disagree.)
