Turn Data Roaming "OFF":
By default the setting for international data roaming will be in the "OFF" position.
To turn data roaming "ON/OFF" tap on, Settings>General>Network>Data Roaming
  - Turning "OFF" data roaming blocks email, browsing and downloads, but it will not block text or picture/video messages.

- When abroad, international roaming rates apply when you send text or picture/video messages.

Utilize WiFi Instead of 3G/GPRS/EDGE:
WiFi is available in many international airports, hotels and restaurants to browse the Web or check email.

Turn Fetch New Data "Off":
Check email and sync contacts and calendars manually instead of having the data pushed to your iPhone automatically. This way you can control the flow of data coming to your iPhone.
    To turn off the Auto-Check functionality tap on
Settings>Mail, Contacts, Calendars>Fetch New Data, Change Push to "OFF" and Select to Fetch Manually

Consider Purchasing an International Data Package:
If you are traveling outside the U.S., purchasing a Data Global Plan will significantly reduce the cost of using data abroad.

Reset the Usage Tracker to Zero:
When you arrive overseas access the usage tracker in the general settings menu and select reset statistics. This will enable you to track your estimated data usage statistics.
    To reset Usage Tracker to Zero tap on
Settings>General>Usage>Reset