In 1995, the Academy of Motion Picture Arts and Sciences created the Visual Effects branch. Since then, a new set of voting procedures has been in effect. This is the current procedure:

20 members of the Visual Effects branch of the Academy
select 7 films for overall consideration.  Then, every
member of the Visual Effects branch assembles, and listens
to presentations made by representatives of each film.
A reel of the shots is shown, and questions are posed to
each effects team.  The Academy members then vote on their
first, second and third choices.  The top three vote-getters
will become the three final nominees for the Oscar. From 
these nominees, the full Academy votes for the winner
of the Oscar.

For more information about the Academy of Motion Picture Arts and Sciences, visit their web site at http://www.academyawards.com.