Forget the diet, forget the Botox. The key to having a successful, even a triumphant reunion, is hair.
If you're a man who still has a full head of hair, you will be a hit even if you are unemployed and have cold sores. If you happen to be bald, be proud of your baldness. Convey the attitude that, "I look great bald, and if you don't think so, you're just wrong." Do not cover up your baldness by wearing one of those things that looks like road kill. For women (and some men) who dye their hair, it's important that the color you choose at least resemble some color that is found in nature.
Don't assume that the people who seemed least likely to succeed have become failures. That thug who was always getting into fights may now be a cop. That guy who always cut classes is now a doctor. And don't be surprised if that girl who always wore too much makeup and was kind of slutty, still wears too much makeup and is kind of slutty, but now's she's a member of the clergy.
Don't cover up your photo nametag and go up to people and say, "Remember me?" Several people did this at my reunion, and all it did was create an uncomfortable moment. I had no idea who they were, and then they were insulted. Finally, I came up with an answer: "Remember you? Are you kidding? You're all I've thought of since high school." Then they'd reveal their ID, and I realized why I hadn't recognized them. They didn't resemble their high school photo in any way — except perhaps being of the same species.
Don't count on romance. Some people go to their reunion, hoping that the person they always had a crush on will still look great, happen to be single, and finally realize that they were meant for each other. If you're going to travel thousands of miles hoping this will come true, you should know that it's possible, but you're a lot more likely to have the airlines lose your luggage.