When anyone overtaking pulls up close behind me I assume they are looking to pass. I simply maintain my speed and when it becomes safe for them to make their pass I move slightly to the right (without vacating my lane). This gives them a hint that it is time to make their move and a much improved view of what is coming down the road. The rest is up to them.
I will blink my lights for truckers (or RV'ers), if they are being pressed by following traffic to indicate I am comfortable with them moving back over after passing. Most trucker appreciate this courtesy gesture and often respond with the "Thank You" signal of blinking their tail lights.
Given the relatively small number of folks who even know what a turn signal is, much less the proper use thereof I never assume/bet my life that they will get it right even if it is blinking. :S