I have spent years defending trad from the encroachment of contemporary because I feel that orginizations like folksong societies need to differentiate one from the other. I have discovered that, while I am still convinced of my position, there are factors existant, today, that were not available when Greensleeves was written. First, the means of instant communication in the modern world make it less likely for a song to be spread, orally. Ten minutes after it is composed, it is recorded, mixed, pressed and promoted. Even in this forum, songs are assosiated with the artists who perform them rather than the culture form which they arise. Still, tradition is an absolute, defined by practice, not by style. Tradition, in music, is no different than tradition in legend or belief. Tradition requires ritual of some sort. "Pomp and Circumstance" is not traditional because it is old, but, rather, because it has become ritual to play it at graduation ceramonies for years. There are newer songs that have become traditional through ritual usage. "God Bless America" is trad while the older, and more beautiful "Stardust" is not. "Sunrise, Sunset" has been a staple at Jewish weddings since Fiddler opened so it is trad while "If I Were a Rich Man", from the same show is not. It's a little early to label "Grandma Got Run Over By a Reindeer" as trad (God, I hope not) but, as you can see, if you want to write a traditional song, you would do well to link it to a holiday. I truly believe that a folk song should be an expression of a culture, not just of the poet.
Traditions, however, need not be more widespread than a family whistle or automobile trip singalong. I understand that families gathering around a piano for a songfest may be rarer than Republican introspection, but, modern though we may be, we still sing to our kids (those who haven't delegated that responsibility to Raffi).
So, if you are interested in having your favorite song become traditional, sing it a lot. Sing it at campfires and, more importantly, get everybody to sing it with you. There can be no such thing as a Bruce Springstien folksong any more than there can be an Al Jolson folksong. It just doesn't work that way. If quality writing was all that was needed for inclusion into "Folk", Matt Dennis would be O'Carolin.