Cool....a thread I started was actually linked to as an example of solid advice.
But all that aside, let me tell you that today we auditioned guitarists, and 2 weeks ago we auditioned drummers, and the tips and advice that was given in that thread really worked wonders. One of the guys commented that ours was the most professional and organized audition he had ever been to, and he is a long-time veteran of the pro music scene.
As for singers, have them sing some of the songs that you'll be playing. Give them several songs and then let them have a go. It's kind of like the scientific method...if you test the same thing each time, with the only variable being the different singers, you will see dependable results each time.