I have a dashcam and the darned thing falls off the screen rather too frequently. Anyone found the magic grail to make it stay stuck?

You can buy an adhesive Dashcam mount from ebay, just make sure your windscreen is spotless before you stick it in place.

3M double sided tape sticks anything to anything. And it ain't ever coming off.

Some window cleaners prevent sticking. Try giving the window a wipe with Windolene first.

Clean both surfaces with Acetone or nail varnish remover.