i know this is an old thread, but i've been working on this a bit myself lately.
I set up a track of guitar or piano or whatever that i'm going to sing with and i put it through my headphones, which i then also feed my microphone through, basically setting up an in ear monitor (IEM) system.
If you've sung and then moved to floor wedge monitors and noticed how much easier it is to stay on pitch, it's an even bigger move with IEMs, I highly recommend it.
I think part of the reason it works is that if you run 'em you hear how you actually sound, not how you think you sound, so you can fix the pitch when you start to drift off. It also explains why everyone uses phones in the studio, and most touring bands are moving to IEM as well (though there are other reasons that IEM > * > wedges as well