I find it helps to have a light touch, and let your finger "bounce" off the fretboard. Also, I have noticed that tap harmonics don't work so well on the lighter, unwound strings (G,B,E).

I'm sure you already know this, but just in case, you need to tap 5, 7, or 12 frets down from the fretted note in order to produce an audible tap harmonic.