* 1 * Though I can't really say for sure where I'd learned this, I'd heard the "mail it to yourself" method is - at best - questionable for proving anything.
First off, just paste something on the bottom of your web pages that says,
Copyright © 2003, Whomever You Are. All Rights Reserved.
Boom. You're copyrighted. (It's like magic.) OK, it's not magic. It's that you've produced something "fixed in a tangible form." Hence, copyright exists.
* 2 * You can make the proper copyright symbol with the HTML © code. (Though it really doesn't matter for legal purposes.) To put on every page, you can just slap it in the bottom of the MonsterCommerce bottom template. (Or you can use templates with a JavaScript file or a server side include. If you don't work with JavaScript or SSIs... don't worry about it. Just use the

included bottom with some HTML in there. That'll work fine.)
* 3 * If you really want to make it official, go ahead and use the other posters idea of burning a CD rom. You can take screen shots or use a web capture program or whatever. Then submit to copyright office. You'll need Form TX...
http://www.copyright.gov/forms/formtxi.pdfand $30 registration fee. You don't have to mail this to yourself and send it to them with any kind of date proof. It's registered the date you send the forms. You can say you made it any date. Probably the best way to PROVE you did something on a particular date is to screen print and get a deck of prints notarized.
* 4 * After doing all this, it will mean little. Unless you have a book or music, (and even then), you're still screwed. Assuming you can find infringers at all, you're going to have to pay for lawyers to talk to them. (Assuming they're even gettable in realms within U.S. law.) And unless you can show damages, you're not going to recover any $$$.
But, doesn't hurt to fill out paperwork and try to do the 'right' thing in any case.
Scott