Before the Hackathon
Preparation is key to hackathon success.
Choosing your team
If you are going as a team, at some point you'll need to form your team before the event. Most events limit the number of members you can have in a team so bear that in mind. Also have a think about having people with the right skills in your team. Do you have enough front end/api/design skills?
Arrive with an idea
I have seen people waste the first few hours of a hackathon determining what to actually do/build. As most hackathons announce the challenges before the day, i'd recommend having a good idea of the challenge(s) you are thinking of attepting and the kind of thing you are considering building before arriving.
Be clear on your reason for attending
It sounds silly but if you are working in a team, it's important to make sure everyone is on the same page as to why they are attending. Is it to learn/try new things? Is it to lay back and not take things too seriously? or is it to commit 110% and win at any cost?
Local development environment setup
Make sure your development environment is setup before arriving. Downloading and installing software during the Hackthon is wasted time. And if the wifi is slow, may be almost impossible.
If you have an idea that uses a technology that you haven't used before, i'd recommend spending some time to at least determine if what you are planning is even possible. Whilst it is not permitted to use code written before the hackathon in the hackathon, there's nothing wrong with figuring out how a concept or technology works beforehand.
Whether you are working solo or in a team, it is useful to already have an idea of how to manage work. I'd recommend at least having something like Trello setup ready to use.
You can never have enough stationary and you can buy/get that prepared before the event. Post-its and Sharpies are useful.
Domain / account registration
If you are thinking of developing a web app, you could always register a dedicated domain name for your hack. Also you could register a relevant Twitter account.
Now that you are all prepared for the Hackathon, check out the tips about what do to during the hackathon.
Make sure you look at the hackathon checklist.
Can no longer make it?
If you can no longer attend the hackathon, make sure you cancel your ticket or place. There may be people on a waiting list.