Shape is one thing, and the better mechanisms will retract the center sufficiently to avoid flattening.
Other things to keep in mind:
Avoid abrading the inside surface of the cup. Never wipe with a dry towel. If/when cleaning it up, always use liquid only (preferably water, perhaps a dash of dish soap) and let it air dry. When storing it, avoid placing the inside of the cup against other objects. Yeah, it's a nice elastomer, but it depends upon zero leaks, and roughing up the surface, especially around the vulnerable outside edges, is an issue.
When storing, avoid locations where any dust could accumulate on the inside of the cup -- if you don't have to clean it, so much the better.
Another huge reason for suction loss -- temperature changes. When possible, install when warm, not cold. If installed 'cold', the increased temperature when taking the device into a hot sunny day will increase the pressure of the remaining air under the cup. That reduces the suction considerably -- and repeating this can cause it to release completely..