I found this on Club Lexus:
I just changed some o-rings on my hose reel, and the old ones (I think the wall thickness is 5/32) were just sitting on the shelf in my garage, when it occurred to me that I might solve my problem with them. So I snipped one, then I rotated the driver side outside mirror fully in (toward the door handle). This lifts up the mirror housing on the shaft (sorta like a screw, I guess), creating a bit of a gap. I stretched the open o-ring between my fingers and slipped it into the gap. Then I rotated the mirror back to the normal position. There is a little bit hanging out on each side that will need to be trimmed. Viola! Shake gone.