The key is to appear to be listening intently while your mind is visiting the Bahamas or whatever, and then at the end of the presentation nod sagely with a small smile of approval. Just don't go to sleep and have your head fall forward and bang on the table....
I've finally realized that most of Dilbert's life is not audible, but stream of consciousness. That is, frames two and three are in their heads. I've never heard of executives that are this honest.