I've been twice once on opening weekend and once on closing weekend. Go on opening weekend way more people and all the swag is still there and the street teams will be giving things away for free too. As for tickets it really depends. I've been able to get free tickets from people who have had people in there party not show up and also by waving a ten dollar bill in the air.
The free ticket one has more of a chance of working more near closing weekend when everyone is burnt out on watching films.
The best advice I can give you is just enjoy yourself. I have as much fun watching the films as I do wondering around Park City. You can meet all kinds of people and sit next to all kinds of people in the theater.