I have also noted that oil had a big role to play in this problem.
If the oil is old, not wet clutch approved, or low this problem can occur..
Clutch adjustment too is a big factor as noted by Gordon.
My bike will do this after riding for an hour or so... to and from work (15min.) its never a problem. I switched to Rotella T synthetic this oil change (still awaiting my cafe seat) I will let you know if this helps my neutral at idle issue.
Lately I have been putting the bike into neutral coming up to stops. Anticipating stops i go into neutral while still moving, works every time.
Note: I do have some play in my clutch basket, and haven't yet micrometered the clutch discs or plates.