4. Get the lay of the land. Observe group formations before choosing whom to approach. Look for people who are most likely to respond positively. These would be individuals standing alone who are waiting for someone to talk to, or groups of twos and threes that are open to new participants. You can see this in their body language: if they are facing outward, chances are they are having a casual conversation and would be happy for others to join in.
Be Genuinely Interested in Others (not just in telling everyone about yourself), ask lots of questions, and smile! :)