Some time ago another fellow Havker asked about them as well, here’s what I wrote then:
"I use my thumb for that: I either twist my wrist a bit outwards or just hold the pick in a way that the very tip of my thumb hangs over the pick slightly and when I pick I let the string touch the tip of my thumb. Basically the same idea as with the 5th, 7th etc fret harmonics: you touch the vibrating string at half, quarter, eighth etc of it’s length thus producing the corresponding element of the overtone series. the only difference is that the harmonics come in a “mirror image” way: when produced with the left hand, the farther you move from the bridge towards the headstock , the further you go in the series, whereas doing it with the right hand above the pickup area, the closer you go to the bridge and farther away from the headstock, the further you go in the series.
You have to experiment with it though, as I found that I need to adjust the technique depending on what guitar/pickup I’m using. Some do well with the slightest of touch, some will sound better when I dig in, too much gain might make them sound quite unpleasant for me etc. Oh, and fresh strings always make it easier. "