The best thing I've found is to nip into your local Ford dealer and ask for the double-sided pads they use for the rear-view mirrors in Fiestas and the like.
You'll just have to cut around the pads to get the right shape for your mirror, but it's a very simple job, and they're pretty cheap, too. ( Get a couple just in case...)
And make sure your screen is really clean.
Good luck.
