What your doing is called natural harmonics. What you can do is try this on every fret and see what frets sound good, and what frets don't ring out at all. Also try tremelo picking the g string, then starting from the 1st fret, slide your finger on the string halfway down the neck.

Matt: IT doens't matter where you pluck the string at, as long as you get the string to ring out really good. Then drop the bar, tap the harmonic, raise the bar and squeal like a pig!

Check this out: http://youtube.com/watch?v=6TpYcA9bSaU&feature=related