I have two things I try to do without fail. The first is always monitor the mirrors for traffic on both sides of the bus. My head is on a swivel. I am constantly seeing who is there and when they disappear I try to understand where they are. Did they get off? Did they move over a lane beyond my view? Did they get behind me so I cannot see them in the mirrors? (There are certain light conditions such as sun shining just right on the lens or in rain when the rear view monitor is useless).

Then when I change lanes I should have a mental picture of who is where, but I still crane my neck and move up and down and side to side to insure nobody has managed to hide. FWIW the way my RH mirrors are set I can see anything in the next lane, from about even with the steer tires on back. Anything forward of that I can see their roof out the entry door window. The ones that are going to get you or you get them are the ones two lanes to the right that move left as you move right after confirming the lane is clear. As a result I try to keep monitoring the mirrors until I own the lane.