Earlier this week, I replaced the light seals on my k1000 and my yashica. I used the black yarn technique and a strip of 2mm foam rubber off an old mouse pad to replace the mirror bumper on the pentax. Its actually an easier fix.
This is the link I followed. http://mattsclassiccameras.com/light_seals/
I remember reading an article where Leica used black wool yarn before using foam rubber. I cannot say from personal experiences, but I have read multiple instances where yarn was used successfully.