I use lane positioning as the first signal - I'll move from the center of the lane towards the right edge.
If that isn't sufficient, I'll wave the driver through.

.: YES I know you are there, YES it appears clear for you to pass; and NO I don't like having cars stack up behind me.

If it's really bad I will stop and step off the road until cars pass.

As a car driver, I appreciate that the cyclist has communicated that he is aware of me and wishes to be cooperative; but I'll decide if I can make the pass safely myself.

On the flip side, if I am on the bike and passing is unsafe I take an aggressive lane position and extend my L arm out and down, fingers spread, palm facing backward.