I’m thinking specifically about the type used to stick things on a shower wall or car window. How does one get a link that is a. strong and b. long lasting? Why is this the case?
I find I have to wet my suction cups and the wall to get the best seal. In addition to the water ‘filling in’ microgaps that could leak, I expect that the cohesive properties of a thin layer of water helps the surfaces stick together.
In a theoretical sense, with a completely smooth and clean dry surface the suction cup would probably stick well. But since I live in the real world and not the theoretical world of frictionless surfaces and round chickens, I usually wet the surface
i’ve had good results in applying a thin bead of vaseline around the outer perimeter of the suction-cup seal. make sure the bead is applied after the seal is established … then run a finger lightly around the circumference … impregnating the micro-fissure gap.
Thanks for the answers! Hopefully now I can enjoy the simple pleasure of having soap at a convenient level in the shower 
So reading the wiki article, I see that it’s apparently the pressure of the air outside that actually sticks it to the wall and the vacuum inside is just there to create the pressure differential? The article could be written in a much clearer way.