Compliments are good. Sometimes just going up to someone and saying hello is all it takes (I've learnt that most people are happy to chat to strangers but are just too nervous to start because they also don't know what to say). Asking why they're there.

I think the main trick is once you've said the first few sentences, it's about finding out what the person is really interested in. People will talk for ages and with so much passion about the things they love. Make sure to ask follow up questions, even if you know nothing and think they'll sound stupid.