Best idea IMO is sticking it on something like YouTube which effectively time/date stamps your song.
However as mentioned earlier, you need to ask yourself why would someone want to take your song? Now it could be that you've written the greatest song in the history of the world, but that aside I think that copyright theft from unknown (ie non published) writers is so incredibly rare that it's practically non existent. The amount of work/marketing/investment that goes into making a 'hit' record is so huge that the potential risk involved in copying another's track just doesn't stack up.
Also, copyright theft is extremely difficult to prove, since it's not enough to say 'their song is a copy of mine'. If anyone stole another song they'd change a few parts, maybe some of the lyrics, a couple of melodies, a chord or two, the production etc. etc. EVEN if you could prove 'Song A = Song B' you'd still need to PROVE that the other artist heard your song and copied it. It's not enough to show that they're the same song as a popular defence is that two people wrote very similar songs completely without knowledge of each other. When you analyse somgwriting and see actually how few subjects are written about (love, loss etc.) and how often the same chord progressions are used it becomes extremely difficult to prove that someone copied another's work.
I think the risk is so minimal that it's not worth spending money on - hence something like YouTube is plenty enough for someone, unless they're really starting to get interest from major publishers, labels etc.