I always walk around my map and think of where I want the most attention (which is where power weapons are placed) and where certain weapons are good at (like BR's by halls and open areas and shotguns in closed areas).
Ok first put power weapons out in the open away from areas where they can be used to over power the person easily, they have to get to the key power spot without getting killed. As for other weapons put them in places where people would usually walk by making it convinent for them. those are my suggestions