Most nations, including the United States, follow the international Berne copyright convention. As a result of this, in the US, since April 1, 1989, anything you write is copyrighted the moment you write it, or, to use the legal jargon, the moment it “is fixed in a tangible form so that it is perceptible either directly or with the aid of a machine or device.” No copyright notice is required.
Of course, you can put a copyright notice on something. Can't hurt. It helps to warn people, and you can also get more and different damages in a legal settlement if your work is infringed, but you still hold the copyright even if you don't have a notice on it.
Here's an example of a proper copyright notice:
“Copyright 2004 by Michael Montoure.”
Alternatively, you could instead use a C in a circle, like this:
“© 2004 by Michael Montoure.”
You can't use (C) instead of ©, though. That's not officially a copyright notice. No, I don't know why not.
If you want to be certain your work is protected, and that you can get the highest damages possible, you can register your work with the United States Copyright Office. It only costs twenty bucks.
Copyright is never accidentally lost. You don't have to worry about losing your copyright by not defending it, as happens with trademarks. You can only explicitly give it away, by selling certain rights to a publisher, or by releasing it to the public domain, which you do by explicitly stating on the document something like, “I hereby release this work to the public domain.” (Of course, your work will pass into the public domain anyway fifty years after your death, but I doubt you'll be much worried about it by then.)
using trademarks
Words can't be copyrighted. Instead, you can trademark them, referring to your brand of a kind of service or product. For example, Apple Computers “owns” the word Apple when used for computers, but Apple Records “owns” Apple for music -- even though Apple is a perfectly ordinary, everyday word. Trademarks depend on their context.
You can't use someone's trademark in ways that would unfairly hurt the value of that trademark. You can't use it in ways that would mislead people into thinking you're the owner of the trademark.
The owner of a trademark can lose it if they aren't seen to actively protect it. “Escalator” used to be a trademark. So did aspirin, cellophane, cornflakes, thermos, trampoline, and many other common words; these words passed into common usage because their owners didn't defend them. You can see, then, why major corporations get so twitchy when people start saying things like, “Would you xerox this for me?” and “Hand me a kleenex.”
So, technically, you should really instead have your characters say things like, “Hand me a Kleenex brand tissue.” Or, instead of, “Do you want a Coke?,” they would say, “Do you want a Coca-Cola brand carbonated beverage?”
The problem, of course, is that no one talks like that. So you're faced with either having your characters exist in a grey, nebulous world where there are no brand names (“Do you want a cola?”), or being technically in violation of trademark.
If you're worried about it, ask your editors, and see if they're worried. If they're not, don't.
fair use
Fair use is a complex doctrine intended to allow certain valuable social purposes, like commentary, news reporting, satire, research, and education about copyrighted works without the need for permission of the copyright holder. It's not an exact doctrine -- the court gets to decide if the right to comment supercedes the copyright of a work on a case-by-case basis.
Important considerations are intent and damage to the commercial value of the copyrighted work. If you're reproducing an article from a magazine in order to criticize the magazines's reporting and the quality of the writing, that's probably fair use. If you're reproducing it so that people don't have to buy the magazine, that's not fair use.
Fair use involves using short excerpts of the original work -- if you reproduced all of it, no one would buy it, and the commercial value would obviously be harmed. You shouldn't excerpt more than is relevant to your commentary. You should also attribute the source of your excerpt.
So far as I know, quoting a few lines of a poem in a story constitutes fair use. Songs, on the other hand, seem to be governed by different laws; you apparently have to get the permission of the owner of a song before you can quote it.
other copyright resources
