One of my favorite tips is to remove the Bump settings from any material that is far enough away from the camera. You'll need to test in your particular scene to discern just how far is far enough. Away from the foreground and some midground area, bumps don't usually contribute much to the image itself (there are exceptions, of course), but they can contribute to a grainish appearance. So if they don't have to be there, just take 'em out. :)