Copyrighting

Jumping in again to say - while sending it to yourself does no harm, it'd not actually an effective protection against theft. Quoted from the UK Copyright Service:

"Posting a copy to yourself is virtually worthless (though we are amazed how often this urban myth is quoted as a ‘way to prove copyright’): As the copy you post remains in your possession, the other party can easily show that you had ample opportunity to tamper with the contents, and of course once opened it could not be used as evidence in any future claim or appeal." (https://copyrightservice.co.uk/register/how_registration_helps)

Also from the U.S. site:

"The practice of sending a copy of your own work to yourself is sometimes called a “poor man’s copyright.” There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration." (http://www.copyright.gov/help/faq/faq-general.html#poorman)

But, like I said, it does no harm - it just won't help much in someone actually fights you

If you have written your novel on a computer and saved it in separate files as you wrote it, you will have the computer files which will be dated as proof it's your material.

I create a new computer file for the chapter I'm working on for each day, divided by month - sometimes end up with 10 files for the same chapter for one month. At the end of the day I save it onto a memory stick. This means I have back up should the computer throw a wobbly and have proof dating from when I first started the novel.

If you're not so keen on technology, Adrian's suggestion is a good one. You can print it, date it and like he said, post it recorded delivery to yourself and store it somewhere safe.