Hi,
I was recently doing exactly thhe same. I agree with the best of the best solution being the USCO, but if you have lots and lots of songs to copyright, this can be very expensive and it takes 6 months.
I think the best way to copyright songs is to use
http://www.provemycopyright.com where you only pay for an account, not per song.
Never use the "Poor mans copyright" as that has never stood up in court, because it's not independent.