Suction cups won't last forever, but here's what's worked best for me. Make sure the cups and windshield are clean and the cups are somewhat flexible (e.g., not hard because it's cold in Michigan). Wipe the cups with a wet paper towel so there's some moisture on the cups. Press the unit into place with pressure on all the cups and hold it in place for about 30-60 seconds.
I've gone anywhere from days to 6+ months with a two-suction cup MicroMan bar in the upper rear window of my SUV with this technique. The trick is to get the suction cups to seat and create a vacuum inside them so they hold, so a good seal around the edge is critical.