Swallow, Don't Spit
USA Today has a list of guidelines for Americans visiting Europe to help them avoid being spat on while they're here.
Avoid American fast-food restaurants and chains.
Keep discussions of politics to private places, not rowdy bars.
Take a rain check on wearing clothes featuring American flags or sports team logos.
Keep your passport out of sight.
Keep cameras, video equipment and maps tucked away.
Soften your speech; Americans typically overshadow their hosts in the volume department.