The 48HFP is a blast. I don't feel like repeating everything that's already been said, but yeah, all the advice given in this thread has been solid.
There are a couple things I'd like to comment on, though.
First, and this probably is pointless for me to say (because you're probably already planning on it), but make sure you go to the screening. And if it's possible for you to watch ALL OF THEM, do. It's so much fun to see what other people did with the same constraints.
Second, I'd like to comment on the brief exchange between Zensteve and ItDonnedOnMe, on the issue of going in with/without any preconcieved story ideas. I've actually put a great deal of thought into this particular issue.
Me? I'm a competitor. Maybe it's because I was raised with four brothers and three sisters. Whatever. I like to compete. And I like to win. But there's one very big caveat -- winning means absolutely nothing if you don't play by the rules. If you cheat, you didn't win.
Now, technically, there is absolutely nothing against the rules of the 48HFP if you want to brainstorm some potential ideas for stories, beforehand. There is NOTHING in the rulebook that says you can't think about it, and even exchange the ideas that you're thinking about, with your teammates. Hell, I've even seen PUBLIC ENCOURAGEMENT from 48HFP OFFICIALS to do just that.
However, I do believe that this act of thinking of ideas, in advance, is against the spirit of the competition. You might not be technically breaking the rules, but you're sort of killing the point of the whole thing.
Additionally, it will be obvious that you just forced the required elements into a pre-concieved story. Whenever any team does this (and it's common), the required elements just seem out of place. But when you build an original story around those required elements, all the pieces just fit. I believe every filmmaker in the audience can tell the difference between the teams that truly follow the spirit of the competition, vs. those who practically wrote stuff in advance.
All that being said, I must admit that I might convert to the Dark Side this year. I'm going to wait until that fateful Friday, to see what genre our team draws, but I'm considering the possibility of a pre-concieved story. Depending on the genre drawn, though, I might choose to stick with what I believe is the spirit of the competition, and create something entirely brand-new, on that day.
Disclaimer: The reason I'm considering a pre-concieved story, however, isn't because I want to get a leg-up on the competition, but simply because this idea that popped into my head would be freaking hilarious, and I couldn't resist the temptation of creating something that I think all the other filmmakers in the audience would be appreciative of.