If the attack is a critical hit, the bonus damage from Sheen is not boosted.
Sheen bonus damage procs off life steal. It does not proc off Spell Vamp, however.
The proc has a cooldown of 1.5 seconds, and the 10 second bonus damage buff does not stack. Using two skills in quick succession will not accrue two stacks of bonus damage, but one.
The bonus damage does not stack with Trinity Force, Lich Bane or Iceborn Gauntlet, since they all have a unique passive with the same name. Only the "best" is used, with Lich Bane being highest, Sheen the lowest.
The proc effect of Sheen causes the champion's weapon or hands glow red, the same graphic as Nunu'sBlood Boil.