Connecting your iPhone to your Mac or PC automatically backs up the information on your handheld, which can be a real life saver if you have to replace it. Restoring that info to a different iPhone is just as easy, but only if you really do have all of the data safe and sound on your computer. In other words, backup your iPhone backup.
Mac users can find their iPhone data backup at Users/Your Home Directory/Library/Application Support/MobileSync/Backup. Windows users should look in C:\Documents and Settings\Your User Name\Application Data\Apple Computer\SyncServices\Local. For example, my user name is Jeff, so I would navigate to Users/Jeff/Library/Application Support/MobileSync/Backup on my MacBook Pro. Make sure to backup the data in those directories regularly so you know your important iPhone data is available in case of catastrophe — meaning you lose both your iPhone and your computer.
Also, be sure to keep track of your email account passwords, wireless network passwords, and the password for your voicemail. You’ll have to reenter those because they aren’t included in your backups.
Restoring the data is simply a matter of connecting your iPhone to your computer. iTunes will ask if you want to restore your data or set up your iPhone as a new phone. Make sure you select the restore option and then click Continue. 
iTunes offers to restore your iPhone data
iTunes will alert you once the data has been restored, and it will also warn you to leave your iPhone connected to your computer. Your iPhone will automatically appear in the iTunes source list after it finishes restarting. 
Leave your iPhone connected after the restore completes
Keeping track of my passwords and backup data saved my digital bacon recently when my iPhone was replaced under warranty. The fine crew at the FlatIron Crossing Apple Store swapped me for a new iPhone to replace the defective one I walked in with, and I would have been out of luck had I not made sure that all of my data and settings were safely tucked away on my Mac.