I use a helmet mirror to maintain situational awareness of what's coming up behind me. My practice (which isn't 100%; sometimes my attention is up front or my hands aren't available..)

• If I see a motorist in the lane next to me change lanes for more separation, I wave to them in a "thanks" manner.
• If I see a motorist approaching in the lane next to me and getting within 100', I swing my head (I don't really look) then I'll lift fingers off left side to acknowledge their presence as they pass.
• If I see any large vehicle (commercial truck, semi, pickup truck pulling a visible trailer) I'll wave to them early 200-400' to let them know I know they're coming up. I yell out either "trailer" or "heavy" to remind myself that there will be more exposure in both length and width, and to be ready for a wind buffet.

I don't know which motorists appreciate this or which ones it infuriates. Most likely 99% don't notice at all. However, these practices make me feel better.