The only thing I do is moisten the suction cup by licking it. Not the most sanitary thing in the world but haven't contracted any virulent diseases yet.
I lay it upside down on a hard surface (shooting bench) with the cup facing upwards and then use my thumb to push the lens down hard on top of it. That squeezes it for a good suction fit without risk of damaging the glasses.
It's stayed on for about 2-3 hours at a time in below freezing temps, that's as long as I usually stay at the range. It will also leave a little ring on the lens when removed but you can take that off with a standard cleaning cloth or maybe a little spritz of lens cleaner. It hasn't damaged the lens coatings at all after two years of use.
