All your work, all your designs, all text and images you create in a job, are under copyright protection!
All drawings that you've made and supplied to the client, all schematics, actually all your paperwork is your work and you should be aware of the fact that this work IS ALREADY copyright protected.
But to make this clear to anybody reading your stuff, it should be labelled in the correct way right from the beginning.
Here some excerpt of a text from the United States Copyright Office. I hope I don't violate copyright protection here :-)
Visit them at: US Copyright Office
The use of a copyright notice is no longer required under U. S. law, although it is often beneficial. Because prior law did contain such a requirement, however, the use of notice is still relevant to the copyright status of older works.
A work that is created .. on or after January 1, 1978, is automatically protected from the moment of its creation and is ordinarily given a term enduring for the author's life plus an additional 70 years after the author's death. In the case of "a joint work prepared by two or more authors who did not work for hire," the term lasts for 70 years after the last surviving author's death. For works made for hire, and for anonymous and pseudonymous works (unless the author's identity is revealed in Copyright Office records), the duration of copyright will be 95 years from publication or 120 years from creation, whichever is shorter.
Use of the notice may be important because it informs the public that the work is protected by copyright, identifies the copyright owner, and shows the year of first publication. Furthermore, in the event that a work is infringed, if a proper notice of copyright appears on the published copy or copies to which a defendant in a copyright infringement suit had access, then no weight shall be given to such a defendant's interposition of a defense based on innocent infringement in mitigation of actual or statutory damages, except as provided in section 504(c)(2) of the copyright law. Innocent infringement occurs when the infringer did not realize that the work was protected.
The use of the copyright notice is the responsibility of the copyright owner and does not require advance permission from, or registration with, the Copyright Office.
The notice for visually perceptible copies should contain all the following three elements:
1. The symbol © (the letter C in a circle), or the word "Copyright," or the abbreviation "Copr."; and
2. The year of first publication of the work. In the case of compilations or derivative works incorporating previously published material, the year date of first publication of the compilation or derivative work is sufficient. The year date may be omitted where a pictorial, graphic, or sculptural work, with accompanying textual matter, if any, is reproduced in or on greeting cards, postcards, stationery, jewelry, dolls, toys, or any useful article; and
3. The name of the owner of copyright in the work, or an abbreviation by which the name can be recognized, or a generally known alternative designation of the owner.
Example: © 2000 John Doe
The author or copyright owner may wish to place a copyright notice on any unpublished copies or phonorecords that leave his or her control.
Example: Unpublished work © 1999 Jane Doe
There is no such thing as an "international copyright" that will automatically protect an author's writings throughout the entire world. Protection against unauthorized use in a particular country depends, basically, on the national laws of that country. However, most countries do offer protection to foreign works under certain conditions, and these conditions have been greatly simplified by international copyright treaties and conventions. For further information and a list of countries that maintain copyright relations with the United States, request Circular 38a, "International Copyright Relations of the United States."
In general, copyright registration is a legal formality intended to make a public record of the basic facts of a particular copyright. However, registration is not a condition of copyright protection. Even though registration is not a requirement for protection, the copyright law provides several inducements or advantages to encourage copyright owners to make registration."
Text from: US Copyright Office
See this link for 'fair use of copyrighted material': § 107 US Copyright Law, Limitations on exclusive rights: fair use / Cornell Law School
The US Code