Also Sorbothane, about the only material specifically designed to dampen mechanical vibrations. However I am talking about mechanical vibrations in the body of the headphones, which arises from Newton's third law, ie. the same amount of energy going out from the drivers is also going back into the drivers enclosure. Aside from Sorbothane there are other companies working on this problem, Grado has a proprietary polycarbonate in its e-series phones, B&W and Audioquest use unspecified material and Sennheiser uses what it calls a "space age" "polymer" in its HD800 (and I would imagine in its ultra costly H2) Sorbothane is a polymer as well, so they could be using it or something similar since its patent ran out about 2000.
Most people think of damping as the use of foam, felt or similar materials to absorb, normally treble in the air. This may be fine if you need/want something between a driver and your ear. That's not what I am talking about here. I don't recommend blutack, it is an adhesive, not a damping material. I made specific comparisons with sorbothane in my thread and it did very little compared to the same amount of sorbothane. However i am sure if you add large quantities it will do more but sorb is better.
If you want to try it, I recommend 1/4 to 1/2 self-stick 70 duro sorb. See my thread in this forum.