Just a bit of common sense should be enough really, I might stay in the middle lane if I see a slow car or big vehicle in the left lane that I know I'll be overtaking in a few seconds, but if it's obviously clear for a reasonable distance that I won't have to be overtaking anyone for a couple of minutes then I will switch back. I have seen it quite often when people stay in the middle when the left lane is completely clear, when that happens I normally overtake them on the right then go straight back across to the left to try and give a bit of a hint that they should move
