The Yamaha AN1x Synthesizer mailing list

> One trick i've found (i'm not sure if other's don't already use it)
is to use  a very slow (value 1 or 2 ) LFO to modulate pitch by a 
tiny amount (again, a value of 1 or 2) of one oscillator and by the
negative value on the other. This adds a slight variation to pitch
that's subtle and sounds rather good!

If you use the editor then the Free EG works great for that purpose. 
You can draw in your own meandering fine pitch instability.