I have made CDs of all my songs and then mailed them to myself..hand-cancelled with the date legible. These get filed in case of the absolutely absurd notion that I might ever need them.
I think this is an old-fashioned idea, probably not technically necessary, and like Oren said: ..whoever has the best lawyer wins.
Speaking of which: ..if somebody steal a song from you, you just have to hope they make millions on it..then a lawyer will jump at the chance to take the case on contingency..(and btw, if a lawyer won't take your case on contingency, you most likely don't have a case).
Wyatt