You must treat your test subjects with extreme care and understanding. You must be patient and reassuring. Test subjects are fragile beings. Be nice!
Make it clear that the user is not the one being tested. Your Web site is. Tell them that they can't do anything wrong. If they can't do something, it's because the site design is at fault, not them.
You must also try not to take it personally when users don't understand what is happening or what to do. Welcome all errors and problems discovered as constructive discoveries.