Every time I've been passed too closely, it's because I've left the driver enough room to do so.
It takes two to tango, and if I'm too far to the right, they'll try to squeak through the gap I've left.
Bingo. Pretty much every state has the FRAP exception for narrow lanes. If the lane isn't wide enough for a motorist to safely pass you while staying in that lane, take the center of the lane. It helps bring them out of their stupor and realize that they will have to change lanes to pass.