Just semi-hosted a hackathon recently. Key elements:

    - good desks / places to sit and code
    - reliable internet
    - free dinner, midnight and/or 3am food
    - healthy food in addition to junky food
    - some like energy drinks
    - don't have people pitch their ideas
    - have an official wiki for people to post their team+idea
    - tell contestants your judging criteria before the hackathon starts 
      (and make sure judges know it)

    - keep presentations speedy, with a gong to keep people honest 
      (<5mins for final demos)

    - have a nice big central room for everyone to code in 
      (don't spread people out across the building)

    - don't require judges to give feedback after each demo

Some of these are lessons learned from the New England College Hackathon (http://nech2011.com/).