As you add contacts to your Apple iPhone, you should synchronise the phone to the iTunes application on your computer, which creates a backup of your phone's data. If your iPhone suffers data loss or damage, iTunes contains the backup of your iPhone and can restore the information as long as the backup is from a recent date. To keep your iPhone and iTunes contact information synced, back up your phone at least once a week.
Plug the USB cable into the iPhone dock connector on the bottom of the phone and plug the other end of the cable into your computer's empty USB port.
Click "Start," select "All Programs" choose "Apple" and click "iTunes" to run iTunes on a PC. For Macs, click the "iTunes" icon on your dock or open the "Applications" folder in Finder and double-click the "iTunes" icon.
Click the "iPhone" tab under the "Devices" section in the iTunes sidebar.
Click the "Info" tab and check the box next to "Sync Address Book Contacts," "Sync Yahoo Address Book Contacts" or "Sync Google Contacts." Click the "Apply" button to sync contacts from your iPhone to iTunes.