See, now I'm going to sound weird...
I sleep with my back to the wall, on the "inside" of the bed. If the Other Half is there, he sleeps between me and the room (the bed HAS to be up against a wall). That way, any axe murderers/crazed knifemen/monsters etc. get him first (sorry love). If he's not there, I sleep in the middle of the bed and must be able to see the whole room from where I'm lying. Perhaps it's some sort of weird primeval instinct. Or perhaps I'm just paranoid. Oh, and the door has to be locked.