Nobody likes to get ripped off, especially if you've put the allotted blood, sweat and tears into a your "dream" script. So, why not get a little insurance that you won't fall victim to the "dark side" of human nature.
A common way to protect your work is to register it with either the Writer's Guild of America (WGA, east or west depending on where you reside) or ProtectRite.
Contact information:
Writer's Guild of America
For west coast:
http://www.wga.org/
or mail to:
WGAW Registry
7000 Third Street
Los Angeles, CA 90048
For east coast:
http://www.wgaeast.org/
or mail to:
WGAE
555 West 57th St
New York, New York 10019
ProtectRite
Now, for added protection, you can also copyright your material with the Library of Congress. Just download your application from the U.S. Copyright Office at
http://www.copyright.gov/ . Then send your completed application, a check for $45 and your screenplay to the address below:
Library of Congress
Copyright Office
101 Independence Ave, S.E.
Washington, D.C. 20559-6000
There are some who have resorted to other ways they may prove ownership of their work, such as mailing it to themselves or merely writing "copyrighted" on their title pages. They're just looking to get ripped off. Try one these methods, get ripped off, then go up against a high-powered studio attorney. Advantage: not you. Stick to the "failsafe" methods. As a matter of fact, go a step further and retain an a paper trail to anyone to whom you send your work. (email and/or snail mail) These are to include all letters, release forms, anything that mentions your screenplay.
Now, there are some who go to the trouble of registering their work correctly and are still paranoid. A little paranoia is good but it should not be debilitating. If you want to get read, get it out there. If you're satisfied with just reading your screenplay to yourself in the wee hours of night, then don't.