From the FAQ:

My suction cups don't stick well anymore, what can I do to make them stick better?
This technique works for me and the suction cups that came with my 8500.
Whenever I find my suction cups losing their grip, I try to revitalize them. I generally have to do this once every 3 to 6 months. Usually, they start to lose grip because they are dirty and can no longer achieve an adequate seal. Sometimes they have become stiff from sunlight and lack good flexibility.
-Remove them from the mount.
-Carefully clean the windshield where they mount and try to eliminate lint or streaks.
-Take the cups and wash them in soapy warm water.
-Then take some water and get it quite hot. We're not looking for boiling, but hotter than tap water (approximately 160*F/70*C).
-Toss the cups in and stir a bit for about 2-3 minutes.
-Dump the water, catching the cups, and flex them about for a minute or two and make sure they have regained their supple feel. If not, try more time in the hot water, or you can try hotter water. Be aware though, that you could easily burn, melt, or otherwise permanently ruin your suction cups. Do at your own risk.
-Use some rubbing alcohol to wipe them down.
-Dry them off, mount them up, and you're ready to go. They should be nice and soft, and just like new again.

Your mileage may vary; I'm not responsible if you mess yours up; I'm not responsible if my process messes yours up; I'm not responsible if this doesn't work for you; Use caution whenever dealing with flammable liquids; Use caution whenever dealing with very hot water; May cause dry mouth, mild headaches, cramping, or oily discharge.

 

I have heard that a bit of Vaseline works well too. After washing them REALLY well, apply a TINY amount around the rim. This will allow them to stay suctioned for longer, because the vaseline seal, as well as the rubber seal need to be ruptured for them to fall. Just my $.02.

john

 

spend $8 and you'll have enough for the next 10 years.

http://suctioncupsinc.com/mcart2/ind...?code=3&cat=12