Look great. Be trim but not too thin, have healthy shiny hair but don't "do" your hair, figure out how to achieve that no-makeup makeup look. Grow an extra inch taller if you can.
Look great without looking like you're trying too hard. You don't want to act as if you got your shit together just for the reunion. What are you, some sort of Romy/Michelle loser? Like you've been looking forward to this moment long enough to remember to make plans for a manicure and pedicure? No, that's for try-hards. Somehow give off the impression that you always look great and that today is like no other. This old thing? I just found it at the Salvation Army earlier today and it just happens to fit perfectly. In fact, I could look even better if I had bothered to go to bed the night before.
Have a nice life. This is, of course, completely secondary or even tertiary to looking great but I guess having a happy professional/personal life is nice too. Figure out how to tell everyone all the awesome bits without sounding like you're bragging. Figure out how to make everyone beg you for the details.
Be self-deprecating and hilarious and interesting and kind. Not that you would do this on purpose, but make people regret not being friends with you way back when.
Don't recognize anyone first. Don't be a dick and pretend not to remember someone, but act like you just have so many friends in your mental rolodex and it takes you a second to recall someone, even if you've been thinking about them once a minute for the last ten years.
Learn how to dance. You will be the hit of the party if you can get in the middle of the floor and impress everyone with your solo dance moves. They'll be all, "Wow, Claire, I mean, that random person, looks so hot, has all that going for her, didn't even know the reunion was this weekend, and has these wicked fresh moves? I am very impressed!"
Do everything in slow-motion, but most especially, the dramatic turn-around. Make sure at least one person will be at the reunion who will spot you through the crowd so you can turn around, slowly, to some dramatic music (do people still use "Take My Breath Away" by Berlin for these measures?) Ideally a big fan will be on-hand for optimum hair-drama.
Act like you didn't even know the reunion was this weekend. You don't want to seem like you care too much. Act like you were just in town for a big meeting and found out about the reunion just a few hours ago. Everyone will be impressed by how busy you are and not-obsessive you are.
Better yet, just don't even go. All the cool people are not going to be there. You don't want to be the loser who actually shows up, right? Inevitably the entire party will be spent talking, glowingly and enviously, about all the people who didn't attend.