A smile can go a long way toward easing tension in a negotiation. Make the effort to smile when you meet the person and to smile throughout the bartering session. It puts people at ease and a positive outlook will make the entire experience more pleasant. Just make sure that you’re not faking your smile.
Most people can sniff out when your smile is fake. Something will just seem off about you, your eyes and nose won’t crinkle the correct way. If it helps, remember that if this goes successfully, you will walk away from the table with something you need or want for less than you were willing to pay for it.