I do a lot of hackathons. My record is 4/26 for winning 1st, 16/26 for winning 1 or more prizes, mentored 3 times and judged once.
Do it on a weekend, have interesting sponsors with prizes, and make the judging/presentation criteria very clear from the start. Also attendance can be hard to estimate, not everyone will show up and not everyone will come back, plan accordingly.
Also, three pieces of advice I always give at hackathons:
1. work backwards from the presentation
2. only build what you show
3. only show the interesting parts