[kenfromm]

Work on the stuff that shows things visually before you work on other behind the scenes parts. Try to get it to show it doing a number of things asynchronously. (A screen that populates a number of actions or data points is going to win out over a single action step-by-step demo.)

Have a screen or two that's nicely designed and laid out. Good graphics, buttons, icons.

Work on your presentation. Many hackathons are beauty contests where a tight presentation and good look will win out over serious stuff under the hood. Have a simple imaginative funny description of what it does. (Funny apps tend to win out over more serious apps.)

Build an app that can get the audience involved. "Everyone take out your cell phone and text ..." "Go to this link and sign in with your twitter credentials." "Take a picture of the person next to your and send it via email to this address."

Do something that makes use of existing data or data that can easily be gathered from users when they sign up.