A couple more tips:

Topical ideas are great. The judges can better identify with the problem that your hack solves, and it seems more relevant.

Hardware hacks tend to steal the show, e.g. a touch-enabled chalkboard, hardware "facebook", or a tweeting microwave. At three of the last four hackathons I've been to, a hardware hack has won. But be wary - at some hackathons they'll specifically tell the judges that the winning hacks must balance being awesome with being useful (specifically the facebook hackathon).

Fake it until you make it. This is a lesson I still need to take to heart. If your app doesn't look like it's going to work and you have an hour to go, hack together something that will give a good demo.