[lcfg]

I'm sorry to say that the "mailing yourself" strategy doesn't work in most countries (doesn't work in the US, for example).

Edit: here's a snopes article regarding "poor man's copyright" with a few links to sources/more information: http://www.snopes.com/legal/postmark.asp

[Navarr]

It would probably work if you could get the poststamp over the content of the work.

But an envelope could be circumvented easily.. Mail yourself an unopen envelope, then put whatever you want inside it later and seal it?