When you say jitter do you mean the rolling interference, through the picture? or do you mean like a harsh vibration.
I have been using GoPro's professionally for years and have had them mounted on various machines. One tip i did learn early on to get rid of the rolling jitter is to use some bits of aluminum foil to act as a pseudo faraday cage.
It worked and stopped some penetration of that nasty roll.
As for mounting vibration, it can vary on the same type of aircraft. I have mounted down the main seam of an R44 with perfect results. People commenting on how stable it is on some YouTube footage then take it to the next machine and its jittering like crazy, so it can be somewhat trial and error.