Preventing Automatic Syncing

You may want to prevent iPhone from syncing automatically when you connect iPhone to a computer other than the one you usually sync with.

Turn off automatic syncing for iPhone:
Connect iPhone to your computer. In iTunes, select iPhone in the sidebar, then click the Summary tab. Deselect “Automatically sync when this iPhone is connected.” You can always sync by clicking the Sync button.
Prevent automatic syncing for all iPhones:
In iTunes choose iTunes > Preferences (on a Mac) or Edit > Preferences (on a PC), click Syncing, and select “Disable automatic syncing for all iPhones and iPods.”

If this checkbox is selected, iPhone won’t sync automatically, even if “Automatically sync when this iPhone is connected” is selected in the Summary pane.

Prevent automatic syncing one time, without changing settings:
Open iTunes. Then, as you connect iPhone to your computer, press and hold Command-Option (on a Mac) or Shift-Control (on a PC) until you see iPhone appear in the sidebar.
Sync manually:
In iTunes, select iPhone in the sidebar, then click Sync in the lower-right corner of the window. Or, if you’ve changed any sync settings, click Apply.