The first piece of advice I can give to people in regards to #fanficfriday is to read the entries for each contest very carefully. A good deal of the questions people ask can be answered simply by reading the entries in question. Here are some of the questions people have asked that are answered in the entry?

Is [blank-fandom] allowed?
~ What is the minimum/maximum word length?
~ What story format is allowed?
        (Short story, one-shot, one-shot collection, etc.)
~ Do the stories need to be new?
~ How do I submit the story?
~ Are crossovers allowed?
~ When do the stories need to be submitted by?

Why must the stories be new?  So far there has only been one time when they allowed stories that were not new into the contest. This was for the first contest where writers could pick any two one-shots within a certain word count to submit for the contest. The later contests were focused around specific themes. They asked for entries that are new so that everybody could have the same amount of time to write their stories in. This doesn't though mean you can't use a story that has been stuck in your head for awhile now, simply that you can't submit a story that was started before the publication date. The story is disqualified if you submit said entry.

Why must the stories be within a certain word length? That is part of the  challenge. Writing within a set of rules may seem like an attempt to stiffle creativity, but in reality it helps to mold you into a better writer. A story is very likely to be disqualiied if you go over the maximum word length -- an exception was made though once because the story was really, really good. How good was it? When I read through the entries prior to the contest winners being announced my heart sank because I thought said story would end up disqualified. Even if the story is really, really good, it may still end updis qualified if there are enough really, really good stories out there. This one really stood out though.

Why aren't crossovers allowed? A good crossover is hard to write, but it can also end up being an easy way to incorporate the theme; one fandom already has the theme incorporated into it's canon, but the other does not. A writer can also take for granted that the theme is already a part of the fandom; this causes the theme to become burried into the background. One of the other traps writers fall into with crossovers is having to large of a cast to pick from. One of Mark Twain's rules for creative writing talks about how each character needs to be there for a reason. These issues are not limited to crossovers, but they are more likely to happen if the writer isn't careful.

Why is the theme important? Let's say that the theme choosen for the contest is "family". When people go to read the winners for said theme, they are looking for stories that focus around said theme. They're not looking for stories where the theme melts into the background and is barely noticed. For example... if I go to read a Christmas fanfic, I expect there to be a focus on Christmas celebrations over everything else. I don't expect the only reference to the holiday to be that the story occured during the holiday time period. Learning to work with various themes is also important as it is one of the five elements to story telling.