How to prepare for a hackathonYou are referring to attending a 24 hour hackathon session or you want to organise it?
Anyway, I will try to answer from an attending point of view.
First of all, hackathons are not an exam where one has to prepare before hand. :) It's more about the passion you have for building something. Sometimes when you want to build something fast, a non stop coding trance can actually help you achieve that. Again, this is not something that you can prepare for. Its the passion that is going to put you in the zone and keep you awake for 24 hours non stop, till you finish that thing.
From being hackathon ready perspective, I can suggest that have a good command over skills in which you are passionate about and have a good balanced team to compliment the ones you don't.
I can tell you the experience we had in our first hackathon. In my point of view, the biggest thing that might help you in a hackathon is going with a clean slate of mind, without any prior intention. This will actually open up a lot of ideas on the spot and will make the whole experience really exciting. When we attended our first hackathon, the Yahoo Hack 2012, we had no idea what we were doing. We were a team of four and had no prior mission to build a particular thing. We were mostly there to experience what hackathons are all about.
The first half of the first day of the event mostly went in talks about different Yahoo technologies. These were really exciting and gave us a lot of insights on how various things can be implemented. Example: someone implemented Siri for the web and we were like wow. Finally, when the actual 24 hour hackathon started, we just had blank faces staring at each other.