Research your story or idea to verify that it is original. Search library databases, the Internet and any other source that might be relevant.
Write several drafts of your story so that the plot and elements of the work are fully developed. Save all drafts. These will show that you worked on the project over a period and can serve as evidence in a legal proceeding if someone tries to steal your work. Save an original copy of your story.
Go to the Writers Guild's online registry website. Upload your work to their database to ensure archival and electronic proof of your writing. The Writers Guild will provide you with a "WGA" number. Place this number in plain sight on your written work anytime you submit it to a potential buyer. There is a fee for this. You also can go tothe WGA office and register the work in person. This establishes the date on which the work was finished by you.
Keep an accurate paper trail via fax, email or certified mail of all submissions to potential buyers or any other outside parties. When you give your story to someone, note the person's name, contact information and the date and time you gave or sent the material. Keep this information in a separate ledger and put in an entry every time you submit.