You just must become one with your office, and then you will be able to notice when someone is coming. I'm damn good at it, I can tell from the sound just when someone is coming to my cube, so I alt tab out of anandtech and into Word (where a convienent document is up and waiting so it looks like I'm working. I've "worked" on the same report for a week now . My boss has a very distinctive footfall, so I always alt-tab into word when I hear it, on the off chance he's coming to see me.
You guys just aren't any good at pretending to work I'd make Scott Adams proud