Curious, are those who have this problem on the larger-than-average size? I ask because we've had 5 Model S cars now and all do it for me to some degree. I asked my wife and she said that it's only me that has this problem. She's an average size smokin' hot babe and I'm a bit of a gorilla.
I know this doesn't help the problem other than knowing the maybe Tesla should have produced side bolsters with more support to prevent them from "giving" too much under pressure.
I did the same after researching fixes for the problem and went with the least expensive "solution" and I can tell you that reapplication is necessary. The Mobile Tech for Tesla had some tube of "expensive German or some such crap" that he applied to one of my first cars shortly after buying it from Tesla and it stayed gone albeit I didn't own the car for a ton of time after application. I think I may order that stuff on Amazon to avoid having to reapply it regularly like I've had to do with the Vaseline.
Another inexpensive option (assuming you have some laying around) would be a small strip of the soft side of velcro/hook & loop applied to the offending area of the side of the armrest. Probably more noticeable though than some transparent lubricant.
