There are several things you can do on the day of the tweetup to ensure its success. It's important to get there early and make sure everything is ready when the crowd starts to arrive. As with any party, you'll want to make sure there's plenty of food and drink, places to sit and easy access to restrooms. Know where everything is located so that when people start walking through the door you can easily give directions.
If you planned the tweetup, then be a proper host. Don't count on people introducing themselves and mingling on their own. After all, if this is the first tweetup you've had for your community of followers, then these people are basically strangers. Get the mingling going by greeting each person when they arrive. As the crowd picks up, introduce one person to another. And importantly -- hand out name tags. Name tags are one of the coolest aspects of the tweetup. The name tags at these events are typically filled out with an @username -- the same way a reply is posted on Twitter. Wear the name tag with your @username, and then introduce yourself using your real name.
There's a lot of talk about how important social networking Web sites will become in the future. Tweetups are one example of how online interactions can turn into face-time. As more people become comfortable with the concept of tweetups, the line between online community and real community may become blurred.