As an author, you have many options in terms of what you can do with your copyrights at the time of publication. Usually, scientists and researchers transfer all rights to the publisher when they sign the publisher's transfer agreement. Once authors sign away their copyright to a publisher, they no longer have any rights to this work and cannot share it without the publisher's permission.
However, it does not have to be all or nothing. There are ways of keeping portions of your copyright that allow you to distribute your work to students and colleagues, post it on the Internet or reuse aspects of it in future articles. To do this, you can attach an addendum to the publication agreement.
Science Commons has created an "addendum engine" that "will help you generate a PDF form that you can attach to a journal publisher's copyright agreement to ensure that you retain certain rights." There are three types of licenses to choose from; access/reuse license, immediate access or delayed access license.
Science Commons / SPARC Addendum
Access - Reuse
You retain sufficient rights to grant to the reading public a Creative Commons Attribution Non Commercial license or similar license that allows the public to re-use or re-post your article so long as you are given credit as the author and so long as the reader's use is non-commercial. (This is a joint offering from Science Commons and SPARC and represents a new version of the former SPARC Addendum.)
Immediate Access: You retain sufficient rights to post a copy of the published version of your article (usually in pdf form) online immediately to a site that does not charge for access to the article. (This is similar in many ways to the MIT Copyright Amendment below) Delayed Access: You also have the right immediately to post your final version of the article, as edited after peer review, to a site that does not charge for access to the article, but you must arrange not to make the published version of your article available to the public until six months after the date of publication.
Learn more about the addendum engine and how to start creating the PDF form at Sciences Commons.