its kinda simple really, its all to do with the way the pick is held and hits the strings. Hold the pick so very little of it will touch the string on picking and also so your thumb (or at least the side of it perpendicular to the pick) will very slightly brush the string as you pick, this creates the dampening effect on the string required to sound a pinch harmonic, or squealer, if you will ;) Remember to experiment with lots of vibrato when playing them!
Note also that pinch harmonics are easier to sound on certain frets then others, ill leave finding those magic notes to you!