They're definitely an advanced technique, but with a bit of patience, you'll get them.
First thing: The correct way to play them is to brush the string with the flesshy part of your thumb. But when I learned them, I was able to devise a much simpler way to play them.
Hold the pick between your thumb and forefinger, as usual. Fret a string, anystring (easier to produce pinch harmonics on lower strings). Once you have the string fretted, and the pick secure, go about plucking the string as you normally would, with ONE difference - very lightly place your middle finger of your pick hand just in front of the the tip of the pic (About an inch or so). Your middle finger may be touching your thumb; that's just fine. When you pluck the string, with your middle finger lightly touching it, a high pitched "squeel" is produced.
That's the easiest way to produce the pinch harmonics. It's critical to understand something else: The string just can't be "pinched" (or brushed) just anywhere. For each fret produces different pinch harmonics at different places.
For example, if I choose the 5th fret on the d string (or any string, for that matter), and I brush my middle finger directly in the middle of the bridge and the end of the neck, a pinch harmonic is produced. However, if I keep my pick hand in that same poisition (exactly between the end of the neck, and the bridge), then slide up to the 13th fret, no harmonic is produced.
So in other words, a different fret position calls for a different position as to where you pluck and brush/pinch the string.
The example above is suited to my guitar, and isn't gauranteed to apply to another guitar. Basically, you just have to get to know where pinch harmonics can be produced between the bridge, end of neck, based upon where you are fretting. Basically, the higher you move on the frets (i.e. 5th fret to 6th fret to 11th fret, etc), the closer you must move your pick/middle finnger to the bridge, and vise-versa.
Also, make sure that you have LOTS of distortion, as this is necessary for the pinch harmonics to ring clear.
And of course, it'll take time to master. Once perfected, you can start using the side of your thumb, as opposed to your middle finger. But while learning, I'd say use your middle finger, without question. And if you feel really comforatable with it - great!! It's not necessary to use your thumb eventually.
I know it's long, but it's a complicated technique, hehehe.
It's worth all the frustration, though, as it makes for one of the coolest effects in all of guitar.
Ladies and gents we're still alive
By the skin of our teeth, now it's killing time
Fasten up your head belts,
Time to ride the stars
It's time to be immortal, 'cuz heroes never die.