Something I do is think of a hypothetical movie scene or situation that the song might be played over. From there, I choose a word or phrase that really highlights that scene and ties it together with the music.

Overall though, I try not to worry about song titles too much because I can tell that the titles of some of my favorite instrumental tunes were just arbitrarily chosen. Songs like Green Onions by Booker T. and The MG's, or Moby Dick by Led Zeppelin. Not to mention the majority of the more modern post rock and electronic I listen to is instrumental, and those song titles are randomly chosen it would seem.

It's a cool thought though - would love to hear how others come up with their song titles!

Cheers,
Deryk