Originally Posted by mr dave

there's also one technique that i learned from an old guitar magazine that can only be done on fretless instruments. harmonic slides. it's not that complicated, just sound your natural harmonic by resting your finger above the string at the correct location. once it's sounding, gently start the slide and move it in whatever direction you want to alter the pitch of the harmonic. the trick is to not press the string too firmly otherwise you'll just sound the regular note. frets would obviously get in the way hehe.