They used to say you could mail the song to yourself and not open the envelope and the postmark would be proof of the date of self-publication. But I've been told by some in-the-know that that won't wash if it comes to a court battle.

You still have to go to a music publishing company to have it registered to you. Organizations like SOCAN and ASCAP don't do that for you. They only deal with royalty issues AFTER you are published.

I guess there's nothing stopping you from founding your own publishing company; many artists have. Sounds like a lot of work and bother to me, but if it's worth it to you, more power to you.