Cheap foam tape from Home Depot, Osh, Lowes, Ace... anywhere. It's usually in the door/window section meant for weather sealing. I used 3/16" thickness and 1/4" wide stuff.
I stuck the tape to the "stationary" piece, usually metal, leaving the plastic trim piece to vibrate against the foam instead of the framing.
The tape is meant to separate the pieces, not to add mass or damp the vibration.