Decide which character you want to be like. It's easier to act like characters you have something in common with. If you're beautiful, rich and haughty, you'll have no problem acting like Massie. If you're impulsive and are always getting a new boyfriend, you should try being Dylan. If you're an athletic soccer player, be like Kristen.
Look beautiful. All of the "Clique" characters are good looking, whether they have cute blonde bangs like Claire or long red hair like Dylan. Wear your hair in an attractive way, read fashion magazines for tips on what kind of make up looks best on you and buy clothes that flatter your body.
Dress like the "Clique" character you want to become. That means heading to the Gap if you're Claire or Kristen or upscale department stores if you're Massie. If you want to be like Dylan, stick with short skirts that show off your gorgeous legs.
Color your hair to be like your "Clique" character, unless you're already a redhead like Dylan or a blonde like Claire. Brunettes won't have to change their hair color to look like Massie or Alicia. A temporary hair tint might make the most sense if you want to switch back and forth between characters.
Get your friends to become the other "Clique" friends. Then you can act out the stories together.