I recently made a trip to see my parents and along the way got to thinking of some polite things my parents used to do for other drivers.
They grew up in South Texas and it is pretty much an unwritten rule that when someone is on your butt, you scoot over to the shoulder and let them pass.
The roads are wide and well paved in Texas, so I can understand if there are other regions where this could not be done.
Something else I learned from them is to 'flash' a semi truck when he clears your front bumper. You tap your headlights on and off really quick, and this acts as a signal to the trucker that he is clear to move over to your lane.
Try it sometime and watch the tail lights of the truck as he sees you. They signal back a 'thank you' by turning their lights on as well.
Not every trucker does this, but there are quite a few that do.
So feel free to add any courtesy ideas here everyone ! 
BTW I had something like this kinda back fire on me though. There was no shoulder on the highway i was on, and this guy was right on me for what seemed like 20 miles.
I moved over to the left lane and applied my brakes ( gently ), thinking he would go past me.
Instead, he became puzzled and slowed down as well. 