From Preventing Automatic Syncing :
You can 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 still sync by clicking the Sync button.
Prevent automatic syncing for all iPhones and iPods:
In iTunes choose iTunes > Preferences (on a Mac) or Edit > Preferences (on a PC), click Devices, 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, connect iPhone to your computer, then press and hold Command-Option (on a Mac) or Shift-Control (on a PC) until you see iPhone appear in the sidebar.
In iTunes, select iPhone in the sidebar, then click Sync in the bottom-right corner of the window. Or, if you’ve changed any sync settings, click Apply.