Many publication agreements require that authors sign away full or partial copyright to their works.
This short video from the Center for Digital Research and Scholarship (CDRS) at Columbia University discusses the ramifications of common publication agreements and gives advice for authors interested in keeping their rights.
You have author rights that apply to all original works
Author rights include:
Signing away your rights
Keeping your author rights
Author rights may sometimes be negotiated. For example, an addendum to the publisher agreement may be attached when publishing a new article so that you may keep some or all of your rights.
Tools like the Scholar's Copyright Addendum Engine can quickly help you create an easy addendum to attach to a journal publication agreement.
SPARC®, the Scholarly Publishing and Academic Resources Coalition, provides a brief overview on how to approach publishers with an addendum, including answers to such questions as "what if the addendum is rejected" at http://www.sparc.arl.org/resources/authors/addendum.