The copyright notice must include three elements: 1) the symbol (c), word Copyright, or abbreviation copr; 2) the year of first publication; and 3) the name or abbreviation of the owner of copyright. In the cases of derivative works it is recommended to indicate a range of years for a work (although this is not required by law). For instance, a copyright notice may be specified according to any of the following variants:

-Copyright (c) Mateo Aboy
-(c) 2007 Mateo Aboy
-Copyright 2007 Mateo Aboy
-Copr. 2007 Mateo Aboy

An exception to the above relates to the copyright notice of sound recordings. Sound recordings embodied on phonorecords are required to include a special notice consisting of the letter P in a circle followed by the year of first publication and by the name of the copyright holder.

Under the Copyright Act of 1909 if an authorized copy of a work was published without notice, the work would be placed permanently on the public domain. This would result in the loss of copyright ownership. This requirement changed when the US passed the Berne Implementation Act of 1998 which became effective on March, 1989. For works published after this date, the omission of the copyright notice does not result in loss of copyright protection. Despite this change in the law it still strongly recommended to use the copyright notice. The notice serves to inform the public that the work is protected and identifies the owner of the work. Additionally, the proper use of a copyright notice makes it significantly more difficult for defendants to claim innocent infringement.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).
[3] Copyright Act of 1976