Some useful info I got from Valentine concerning their suction cups.
"Be sure to clean your windshield with a non-soap cleaner; one with ammonia. Soap leaves a film. Use inexpensive paper towels or newspaper. More expensive towels contain lanolin that will leave a film. Do not moisten or lick the suction cups because the next time the cups will not stick. You can rub your thumb around the cups before attaching. When you attach the cups to the windshield, get the suction and then tug back on the detector to increase the suction. Take your suction cups and drop them in almost boiling water for just a few seconds. This will restore the shape."