Like willakz said, after you fret a note, use your pick hand and allow the edge of your thumb to touch the string at the same time the plectrum hits it when you pick it. To get that squeal of a pinch harmonic, you may have to really choke up on the plectrum and pick the string within the v formation that is made by your thumb and pick. You might also try various positions on the string you are picking between the bridge and fretboard. If you keep experimenting you will eventually get it.