Initially, the author of a work owns all rights to that work unless she is otherwise contracted. If the work is created under a "Work for Hire" type of contract, the author is not considered the owner of any rights to the work. The company holding the contract owns all rights and the author cannot sell or resell any of them. If a more limited contract is signed before the completion of the work, the author's rights are modified by the terms of that signed contract.
If you intend to resell a work after its first sale, make sure your contract only sells first rights. This is generally termed First North American Serial Rights, First English Language Serial Rights in the U.S., for short works, or First North American (or English Language) Rights or First World Rights, for longer works. Contracts containing the term "exclusive" mean you cannot sell the work to any other market for the amount of time specified in the contract. This often ranges between three months and three years for short works and may be five to ten years for longer works. Contracts that specify "nonexclusive" rights allow the author to resell the same work without restriction.
Whether the contract specifies first rights or not, look for the term "Reversion of Rights." This specifies when rights to resell the work revert to the author. Because book publishers sometimes change personnel or take on more books than they can publish, your book may be contracted but never printed. A Reversion of Rights clause in the contract specifies a time--often two to five years after the contract signing date--at which the rights of the unpublished work are returned to the author. Once this clause expires, the author (or other rights-holder) is free to resell the work as desired.
Some publishers ask for all print rights in their contracts but allow authors to resell certain rights, such as reprint rights, upon written request to the original publisher. If you want to resell a work that was previously sold and you haven't retained the rights you need, send a written letter to the publisher requesting reprint rights. These are often granted with the stipulation that the new publication includes a note stating where the article or story was originally published. You can see examples of this in books and magazines that contain notices such as "This article originally appeared in X magazine. Reprinted with permission." Resale rights may have additional restrictions, such as splitting a percentage of the resale income with the original publisher or only being available for resale in markets that don't conflict with the original market, such as selling an article to a magazine specific to the Pacific Northwest, then reselling to a local New York area magazine.
In today's publishing world, print rights are not the only rights to sell, so thoroughly read contract clauses on foreign language, multimedia, new media, digital and audio rights. Unless a publisher has the capability to produce your work in other media, consider negotiating to retain these rights so you can sell them to other publishers or producers without having to renegotiate your contract. Screenplay options, video game options, audiobooks, podcasts and e-books all fall under additional rights. If you do not retain these rights in your original contract, you may still be able to resell these rights but the publisher may be entitled to a portion of the income, often around 20 percent.