Defending screen passes is tough. Good offensive coordinators are setting you up all game long to hit a big screen play. It's a great way for them to get an athlete in space who's hard to bring down, too.
The secret to defending screen passes starts with removing that word. Stop thinking about a screen pass as a pass.
Screen plays are an extension of the run game. They are used as a constraint play for the passing game. Specifically, screen plays are a constraint against your aggressive pass rush.