Before you sent it out to Producers, Production Companies, Broadcasters, Distributors or whatever, it's advisable to submit your work to be archived by the Writers' Guild of America (or The Writers' Guild of Canada) Intellectual Property Registry and document your claim to authorship.
Registrable material includes scripts, screenplays, teleplays, treatments, synopses, outlines, ideas, stageplays, manuscripts, novels, short stories, poems commercial, lyrics and drawing - and probably a lot of other stuff that I haven't mentioned.
Everything that you need to know to register with the WGA (USA) can be found right here. The fee is $20 for the general public and $10 for WGA members. You can do it electronically and I highly recommend doing it.
Likewise for us Canadians - you can register here and it only costs $20 for members and $35 for the general public... WHAT? What a frickin' rip off! You're better off sending it to the US registry. Hell, that's where I send mine.
You can also do the Library of Congress thing - but there's more paperwork.
Now in addition to that - if you really want to be evil about it, do what I do. Whenever I write a pitch for a project, a pilot script or novel, I build a "canary cage" trap into the creative. This comes from reading too many Tom Clancy spy novels. But I really do this - its fun, easy and devious:
Imagine you're in a court room. Someone has ripped off your dream project. When they're on the witness stand, you ask them to read a paragraph from "their" work. They read it. Then you ask them to read the first letter of the first word in the first sentence.
They read it out. "S"
Read the first letter of the second word. "T" Third word? "E" Fourth word? "V" Fifth word? "E"
Of course I use something far less obvious than my name (or do I?) - but its always hidden away in there and I'm waiting for the day I can spring it on some unsuspecting evil doer... I hate to say it, but getting ripped off by a major corporation or even better some government agency - and being able to prove it in court, is like winning the lottery.