Hold your finger lightly on the string, so that it is barely touching it. Then pick the string and immediatly lift your finger off.. that's how you get those cool-sounding natural harmonics. Oh, and incidently, it's not just the 7th, 9th and 12th frets where you can do this. You can actually do it on the entire fretboard, it's just damn easier on the frets mentioned above. Try going through all the frets and seeing what you can get out of them. You might be surprised.