Smile - it is welcoming to other people. Make eye contact (and learn when to drop it) - it connects with the other person.
Be careful with your language - saying "It is X" comes across very differently to "I think it is X".
Open your body language - hunched, closed posture looks defensive. Standing so the other person can see across the room gives them an out - even if they don't want to use it, having it makes a big different.
And, most importantly of all, listen more than you talk. People love to feel like they're important, that their interactions with the rest of the world mean something. If you truly listen to them and take on board what they're saying they will notice it - because so often we just get talked at and what we're saying is ignored.