The pinch Andrew detailed is correct. Most guitars however have a "sweet spot" for each string, making this happen more consistently. This has to do with scale length, tuning pitch, and induced string pitch (tremelo and bends).

Fret a G string on the 7th fret for this example. Without bending the string or altering pitch with a tremelo, apply the pinch harmonic (as Andrew described) on different areas of that string over the body. Most strat style guitars will respond better with the pinch applied near the middle or neck pickup. LP's tend to respond better across the bridge pickup to near neck pickup. Find the sweet spot and these guys will show up much easier. You may find more than one area that responds as well. It all depends on your axe.

If you have a Floyd - Brad Gillis (Night Ranger), Joe Satriani, Via and others have used this effectively by combining left hand harmonics and pinch harmonics with tremelo bends (up and down). If you begin a pinch harmonic with the tremelo depressed (lowering the note) with the intention of raising the tremelo (climbing scream), this reduces string tension initially and may change the location of your "sweet spot" for that string (usually move closer to the bridge).
Experiment and *#@&ing rock!