Have a story worth telling. Take it through multiple drafts, revisions and editions before sending it out to be reviewed. Have a very tight circle of reviewers so that there isn't an attempt to please everyone with multiple commentators.
Accept criticism. This is the hardest part of the writing process, but necessary. The part that is loved by the writer, may be the part impacting the pacing of the story.
Say your sentences out loud. Lines that type out easily may be awkward in speech.
Know your limitations. When it comes to fan productions, its easy to imagine grand sequences and battles. But, rarely are fan films written by professional writers, and have access to professional actors. So, know that your actors might need some coaching in order to sell the scene. Limits are good but there needs to be an awareness of them.
Finally, have fun.
This is just from personal experience. I'm not a writer, producer or actor by trade or training. But, I've worked on enough fan productions that fizzled out because the dreams were too big.