Don't sign your rights away!
Traditionally publishers may ask you to assign exclusive copyright when you publish with them. For commercial purposes publishers need only the right of first publication of your work, and normally do not need to control how your research is used and distributed beyond.
The Australian Copyright Council asserts that
The general rule is that the "author" is the first owner of copyright if the work is a text work, music, a dramatic work, a computer program or an artistic work (such as a drawing). For the purposes of copyright law, an "author" is a person who gives expression to the ideas or information in a work
The University of Sydney encourages researchers to negotiate their publishing agreements, to retain author rights and control. As author and copyright owner you may not need to assign exclusive rights to a publisher. Non-exclusive license agreements allow more flexibility in the use of your research.
You can make your publisher aware that after a certain time has elapsed from initial publication, for example after twelve or eighteen months, you intend to self archive your work in the Sydney eScholarship Repository. You would grant the publisher a non-exclusive license to the first publication of your work, but you retain the right to distribute it after that agreed time. To assist the University of Sydney Library has developed an "Author Addendum" tool which can assist in negotiating your agreements. Retaining your rights does not adversely affect the commercial interest of the publisher.
For a comprehensive account of how to go about this see section 7.5 - Working with your publisher in of Pappalardo, K. (2008). Understanding Open Access in the Academic Environment: A Guide for Authors
Two studies, conducted by the Queensland University of Technology's OAKlaw Project (2008) and the Sherpa/RoMEO Project (2001), examined publisher copyright transfer agreements, finding that between 85% and 90% required the author to assign exclusive copyright to the publisher. Moreover, these studies discovered that many academic staff don't know why they sign away their rights.
Where possible, it is important to retain your copyright. Many funding agencies support Open Access to scholarly literature, making it an imperative to preserve control over the future use of your work.