Some journals make available a standard 'Transfer of Copyright' form from their homepage. This is a PDF document that journal contributors can print off, complete and mail back to the address given.
For those journals for which it is available, the 'Transfer of Copyright' form can be found in the 'Journal Information' menu that appears on the left-hand side of the journal homepage and throughout all its related content and information pages.
It is common practice for journals to acquire copyright in all contributions. There are two reasons for this: (a) ownership of copyright by one central organisation tends to ensure maximum international protection against unauthorised use; (b) it also ensures that requests by third parties to reprint or reproduce a contribution, or part of it, are handled efficiently and in accordance with a general policy that is sensitive both to any relevant changes in international copyright legislation and to the general desirability of encouraging the dissemination of knowledge. You can find more information about transfer of copyright on the form itself.