As mentioned, the Berne Convention means anything that can be copyrighted (creative works: stories, articles, art, etc. - but not data or ideas) are automatically copyrighted.
But in US law, if something infringes your copyright, you can get more damages if the court rules it was "willful" infringement. I'm not totally sure it's settled that a copyright notice makes a difference, but I'm guessing big companies think it can't hurt. (Also keep in mind whether you're as willing as big companies to go to court, or if the most you practically want to do is have a lawyer send someone a polite but firm letter.)
Another thing you can do if you want things to go better in court (and isn't really worthwhile otherwise) is to register your work with the Copyright Office.
But in US law, if something infringes your copyright, you can get more damages if the court rules it was "willful" infringement. I'm not totally sure it's settled that a copyright notice makes a difference, but I'm guessing big companies think it can't hurt. (Also keep in mind whether you're as willing as big companies to go to court, or if the most you practically want to do is have a lawyer send someone a polite but firm letter.)
Another thing you can do if you want things to go better in court (and isn't really worthwhile otherwise) is to register your work with the Copyright Office.