Registration of Copyright is the best way to provide protection. Copyright a whole book of 'em for one fee, and then, if asked by someone to allow use of one song, register that single song for the fee, again. The reason for separating a song is that it makes searching easier, and if someone wants to use your song, charge them for the extra work to copyright it.

DO NOT RELY ON THE "MAIL A SEALED PACKAGE/ENVELOPE" SYSTEM. All it takes is one sleight-of-hand magician to show that whatever you put into that envelope may not have been what you claim it is. If it comes to a jury, and your opponent can show that you may have been able to substitute anything in that package you brought in to show your ownership, and blam! The jury votes against you and your song is gone. Don't ever let it get as far as a courtroom! It's just too chancey.

Bob