1) Arrive Thursday, leave Monday evening. (I leave Tuesday morning, but Monday night is DEAD DEAD DEAD!) You will see more people in costume on Thursday evening than you will all day Monday. Badge pickup on Thursday evening. Saves you time!
2) Don't try to do everything. Pick the things you want to see most and wait in lines. (Anything in the big rooms or with major stars will have lines...but fan panels often have no lines.) If you stay in one of the 5 con hotels, you will be able to watch Dragon*Con TV and see the major panels LIVE on TV in your room! That's often a better option than going to the room, but it's kind of hard to figure out if a panel will be on D*C TV or not.
3) There are 3 dealers' rooms, artists' alley, and an art show. Don't just visit 1 dealers' room and assume that was it.
4) Cosplay! Like, OMG COSPLAY! SO MUCH OF IT! In the evenings, everyone converges on the Marriott. You should too.
5) Don't try to get a drink at the Marriott. They're overpriced and lines are stupid there.
6) If you're not in the parade, watch it. It's awesome!
7) Walk through the Walk of Fame and marvel at all the famous (and not-so-famous) TV and movie starts in one room. Even if you're not an autograph hound, it's cool to see all those folks. ...and if someone doesn't have a line, go shoot the breeze.
8) Everyone eats lunch at the Peachtree Center Food Court. ...so eat at off hours if you're eating there. DO NOT go there at noon or 6pm.
9) If there is a Big Damn Shindig done by the Whedon programming track, GO! Be prepared to dance.
10) Take photos, meet people, and have one hell of an amazing time!