We're told that Sauron could never assume his fair form again, it having been destroyed in Numenor. He must have had some physical form because Isildir is said to have cut the Ring from his finger. And Gollum does say that Sauron only has nine fingers. Does this mean that he had other forms lying in wait for him in Mordor? Were they like meat suits that his spirit could throw on when the need presented itself? So the fair form was destroyed, but he had the Dark Lord form hanging in his necro-closet and when his spirit returned to Middle Earth, he slipped on the Dark Lord form to continue his work in Middle Earth.
I know that I'm being slightly tongue in cheek here, but it appears that the destruction of the physical form in a monumental catastrophe such as the destruction of the Two Trees or the Downfall of Numenor , seems to remove that form forever from being used by the spirit of the Maia or Vala, while a mere physical death in that form allows that form to be reconstituted. Thus, when Sauron is defeated/killed by the Last Alliance, he was able to flee that form and then use it again later, albeit with some of wear and tear still embodied (no pun intended).
Although, it raises the question why Sauron was able to reconstitute himself in another form after 'death' while Durin's Bane and Gandalf (and Saruman) were not able to without permission. But that's for another thread....