Being part of two startup weekends in Sofia, Bulgaria, once as a teammate and once as a leader, I've managed to make some conclusions. They are solely based on my personal experience and on my talks with other participants. Both of the times, I've been unsuccessful with the projects I've worked on, or at least that's how it felt at the end. But as every entrepreneur-beginner knows, failure itself is a small success, because it gives you the opportunity to learn from your mistakes.
Always prepare and practise. It doesn't matter if it's your pitch or final presentation, things will go far better if you practise speaking in front of an audience and structuring your speech for the time you're given. If you're pitching an idea, you must have a particular problem in mind or well-defined concept for a solution. Sometimes only the idea of a problem makes people want to team up and find a solution.
Best projects are either from:
Teams who have a passion in common, like music and hardware, because the passion unites them and shortens the time of the team building. Still, you should be careful and get to know the teammates well, because they might have a common passion, but still have different mindsets.
Teams who have a pain in common, and this pain is quite specific. Chances are they have a lot in common, which will make them great teammates.
Or sometimes teams who have worked on their project before the weekend, because teams are well defined and if you don't fit in, you'll know it the moment you finish your first talk.
Big projects like web portals and complex social networks from scratch don't stand a chance, since the definition of the idea is a process which takes a lot of time and there is no way to simultaneously build and define an idea for 54 hours. Idea definition consists of all the parts of the business you're starting - the business model, the customer validation, and the overview of your technical implementation. A clear, but simple idea is always better than an unclear MVP for a complex system.
The most important thing is team building. At least half of the time you're going to be team building. If you do not, you probably won't get the results you're looking for, because there will be obstacles for every decision you make. Slacking with the team is something good, but only as long as you have enough time to build your idea. If you don't have enough time, don't be disappointed with the resulting project. Still, if you have fun, you're guaranteed to have made great new friends.
Never pitch your best idea if it's not your only idea. Don't be afraid someone will steal your idea but remember that sometimes it might be better developed without the restrictions of the event - the limited time and resources. The Startup Weekend is a great channel to find cofounders and people who share your pain or passion, so don't miss the opportunity to share it, as long as you feel you're talking to the right person.
Also, carefully select your team. If your idea is well-defined, the people who are interested will join you before the announcement of the team leaders. Check up in front the experience of every one of your teammates, because the time frame of the Weekend offers little time for learning. Do not start to do something you don't know how to achieve - ask someone else to do it instead.
If it depends on you, always make a decision, but never impose only your opinion. If you can't decide, ask. There will probably be people who have been through the same problem and have found solutions that might help you in the same room. Never say no to people asking for help. There is always a field they are capable of helping you afterwards. Don't forget to exchange contact information, so you remain connected.
If you can't make a working product for the 54 hours, fake the experience - build a static prototype or a good mock up. Don't say you have something you don't, because nobody cares if you do your startup, so there is no point of trying to lie to people.
Listen to what the judges tell you. Most of the time, they have a point, but if you can, try to talk with each one of them individually. That's the time you'll get the chance to see the big picture. During the Q&A session after your presentation their comments will mostly be negative, but constructive. If you can't answer them, probably you've got something wrong with your project. But if they don't approve your idea, that doesn't mean it sucks. You can still try to get it done, but if things don't feel right, you should move on. That's the time when you naturally generate a better idea, so don't be blind for it.
If you don't succeed, don't get demotivated. Always look at the bright side of life and evaluate the whole experience, not only your project.