chazza is spot on, good eye contact, freindly smile, light freindly greeting. If you are too shy or find eye contact difficult then don't make direct eye contact, just look at the bridge of their nose, they will not be able to distinguish the difference and it's very much easier than meeting people's eyes if you are shy. Think of something pleasant to say that you will have in common with this person in advance if possible to get the ball rolling and ask about them ( people always like to discuss themselves) but intersperse any questions with chat and info about yourself too so it doesn't come off as an inquisition. Don't fold your arms (negative body language) and stand just a little off directly opposite them as dead ahead is subconscious aggression. Listen attentively to what they have to say and show a genuine interest.
Small talk is more work than you think but there is a formula at least for not appearing churlish or bored. When you leave them always smile and say how nice it was to meet them.
Good luck.