Yeah, stole that from Hitchcock but he did it first! Philip Morton's blog has some great stuff on editing and while I can't recall ever shilling somebody else's blog outside the right column, I highly recommend you read Philip's latest post in its entirety several times. This is only the Cliffs Notes version of what to look for as you edit your screenplay. Don't forget to look at the replies for the explanation of #4 from Bill Cunningham.
(1) Trim excess setup, exposition, & dialogue
(2) Cut out repeated action
(3) Multiply action to condense the story
(4) Avengers' technique
Billy Mernit also has a great post today about technique that's worthy of your refrigerator door. What a marvelous thing this internet is!