Many faculty are surprised to realize they may no longer own copyright to articles they have published. Be sure to read the fine print on any contracts you sign as you publish your works. Frequently that paperwork includes a transfer of copyright whereby an author may sign away all copyrights to the publisher. This means the author no longer has the right to copy, display, or transform the work. Many publishers are now sharing copyright with the author, creating a balance between the publishers needs for copyright to recover costs incurred during the publishing process and the author's rights to reuse and share their own works. The SHERPA site (see box on the left side) provides a snapshot of different publishers' policies.
Increasingly, government agencies such as the National Institutes of Health (NIH) require a copy of research results be posted in an open access repository so that the taxpayers who fund the research grants can gain access to the results. See this helpful guide on When and How to Comply.
SPARC, the Scholarly Publishing and Academic Resources Coalition, provides this resource on Author Rights which includes a sample addendum you can submit with your publication, indicating you wish to retain copyright.
GW Libraries page on Author's Rights.