Start with a clean dry surface. If the temperature of the suction cup is drastically different than that of the windshield it might cause the suction cup to lose its grip. If you need to, a little moisture might help it to seal firmly. Find your target on the windshield, press it on, and apply firm pressure to the center of the mount. What you are doing, and want to do, is to push all of the trapped air between the mount and the windshield out from behind the mount. Try not to let the mount slide (if it slides easily you might have used too much moisture) and keep firm pressure for about fifteen seconds.
