How to Turn Off Auto Sync on an iPhone

Setting up your iPhone to work with a new computer can completely erase your data from the device unless you are careful. Disabling the automatic sync option in iTunes is an essential step to ensure your data is preserved. After the auto-sync option has been disabled, you can safely connect your iPhone and begin the task of copying your music, contacts and applications over to the new computer.

...
Canceling the initial sync on a new computer will preserve the data on your iPhone.

Step

Connect your iPhone to the computer using the USB cable provided with the device.

Step

Open iTunes and wait for your iPhone to appear in the list of connected devices.

Step

Slide your finger across the bottom of the iPhone screen to cancel the syncing process.

Step

Click the "Summary" tab in iTunes and uncheck the box for "Automatically sync when this iPhone is connected."