this is something cool i figured out how to do. it works with artificial 
harmonics.

basically you play an artificial harmonic and slide the fretted 
note whichever way you want it to go. sounds very cool on a fretless 
bass :)


try holding down the 12th fret and putting your index finger 
of your right hand under the string like you are going to pop the 
string. put your thumb over the 24th fret and pull your index 
finger up and pull your thumb off the string.
|——12&24————————————————————————————————————————————————|
|———————————————————————————————————————————————————————|
|———————————————————————————————————————————————————————|
|———————————————————————————————————————————————————————|
it should make a high pitched harmonic.

now try it again but after you make the harmonic slide your left 
hand up the string
|——12/&24———————————————————————————————————————————————|
|———————————————————————————————————————————————————————|
|———————————————————————————————————————————————————————|
|———————————————————————————————————————————————————————|

if you do it right you should make a harmonic slide :)
~it sounds really cool on a fretless bass~