I have to say I never bother to send a song in the post to myself..
For one thing I am sure I would completely forget I had posted something to myself and open it.
I also don’t really believe there are a whole bunch of people out there ready to steal my tunes to make a mint with. Although I would love it,if that was true.

But I often talk to musicians who are really anxious to protect their material.
and it seems like there are a whole bunch of websites out there playing on that fear.
Many of them warn against “poor mans copyright” and say it is worthless.
As mentioned above that’s when you send a copy of your cd or lyrics to yourself in a sealed envelope. In the event of a dispute you would take the unopened packet to court as evidence of when you had completed the song, there is clear proof of the date as post is time stamped.

Perhaps its true that poor mans copyright doesn’t hold much clout in the U.S. but according to the advice from the IPO, the official government body responsible for granting Intellectual Property (IP) rights in the United Kingdom, this method is totally valid
 here in the UK.

“There is no official registration system for copyright in the United Kingdom (UK) and most other parts of the world. There are no forms to fill in and no fees to pay to get copyright protection. So long as you have created and fixed, for example in writing, an original work that qualifies for copyright protection, that is it falls into one of the categories of material protected by copyright, you will have copyright protection without having to do anything to establish this. It is a requirement of various international conventions on copyright that copyright should be automatic with no need to register.

To help protect your copyright work, it is advisable to mark it with the © symbol, the name of the copyright owner and the year in which the work was created.
Additionally, a creator could send himself or herself a copy by special delivery post (which gives a clear date stamp on the envelope), leaving the envelope unopened on its return (ensuring you also know what is inside each envelope in case you do this more than once). Alternatively you could lodge your work with a bank or solicitor. It is important to note, that this does not prove that a work is original or created by you. But it may be useful to be able to show the court that the work was in your possession at a particular date.” www.ipo.gov.uk

The songwriters guild point out that it is very important not to use sellotape or staples on the envelope to prevent any claims that the packet could have been opened!

However if you are still not convinced
The Songwriters guild offer a free unlimited copyright service for its members (membership is £55 per year ) www.songwriters-guild.co.uk
I also found a site http://copyright.co.uk/ where you can register the copyright of any piece of work for £31.00 per item. (this could get expensive really quickly)
Or you could consider joining  PRS, the performing rights society.

You can register your songs on their database and they will collect money on your behalf when your song gets played on radio or performed in public. www.prsformusic.com
Extra info
This sensible bit of advice is also from the songwriters guild.
How to protect your copyright when sending out demos and promoting material..
”The main priority is to ensure that you keep a copy of the letter you have written that you are sending with your demo, and to ensure that you get a certificate of posting with the name of the person/company, to whom you are sending your copyrights. Evidence that your work(s) is in circulation is very important. Always use the copyright symbol © or the word copyright on your lyric sheets and recordings along with your name and the year date. Ensure that you label everything (lyric sheets, manuscripts, recording media i.e  CD, DVD, Video etc and their cases) with your name, address, telephone number and email contact.” www.songwriters-guild.co.uk

I reckon its much more important to have a written agreement with co-writers.
The Musicians Union have a good simple song split contract that consists of the names of writers – song titles –agreed share of each song, dates and is countersigned by a witness.So you could easily design your own.

So much better than blood and tears being shed later down the line.