People will start staking out front and center spots by 5 p.m. Some people claim that, depending on the crowds, you can get pretty good ground spots as late as 7 p.m., although not front and center.
Some families spread a blanket, bring a book, some food, and a cushion to sit on, and the rest of the family goes and does rides while one person holds down the spot. I would be sure to have everybody back and sitting by 8 p.m., though, because people will start crowding in on your territory and it will get very tight before the show starts.
Other people claim that you can get standing spots behind the middle light pole, which rises up in the air just before the show starts. I think a lot of people are in on this secret, though, according to some recent posters.
Good luck!