One of my friends dressed up as Jane. She put her hair up in a bun, wore a simple black dress with a cape and knee socks just like Jane did. She also covered her face and arms with light colored makeup to give her that pale vampire look. She also ordered red colored contacts online to achieve the Volturi eyes. It looked great!
If you don't feel like dressing in costume you could also wear your favorite Twilight shirt and show your love for your team. Doesn't matter if you're Team Edward, Jacob, Emmett or Jasper... all Cullen's and vampires are loved here! Shirts can be found just about any place Twilight merchandise is sold.
Another one of my friends dressed us as Breaking Dawn Bella! She wore her favorite Twilight shirt and "Cullen Eyes" that she got from the Twilight Convention. She was pregnant at the time of the party and she drank out of a styrofoam cup with a straw just like Bella did when she drank the blood in Breaking Dawn. Fun!
If you don't feel like dressing up or you can't decide which one is your FAVORITE shirt you can always wear Twilight colors such as black, white or red. To make it fun you can add a Edward or Jacob button to your shirt and tie a red satin ribbon in your hair to be like the Eclipse ribbon. Ribbon can be found at any craft store and buttons anywhere Twilight merchandise is sold.