I treat these sort of social environments like bungee jumping. The pure-stranger environment is a rough start. It's nice to have familiarty with some of the people there. In a college environment, know some people there and which classes they are in. I socialize in a work environment, and listen over time for interests that people have, and go from there (if my interests match). I try to absorb facts about people to leverage conversation down the road.
I would assume you have some idea of what sort of people are going there, and what they may be interested in.
This are my two steps to prepare, by no means a solution for everyone:
-Go in prepared with things to talk about (a wide range of topics you have some knowledge about, Wikipedia in advance? )
-Listen to what people are talking about, I don't know when it is appropriate to barge into a conversation, but if there are 3-4 people engaged, it isn't a bad time to sweep in with a comment and see how it works
Once prepared, jump in and do it, or run in fear. I go either way depending on how people are reacting.
You just have to poke around and see what topics work for various people, never simple, but when you find something in common it can actually be quite fun for short periods of time.
When it becomes overwhelming or exhausting, try to recognize the signs and find an excuse to leave early before shutting down if that ever happens to you. It leaves things on a positive note for a return visit. I always have my cell phone on vibrate, so a fake received call works for me (yeah, it's a lie, but better than walking out of a conversation abruptly).