A long time ago when I used to be able to go to the SEMA shows in LV (early '80s), a couple of engineers at the STP booth who at the time were making a competitor to ArmorAll, I do not remember if it was the "Son of a gun" BB73Challenger mentioned, said to not use any such product on the dash because it would cause a failure cycle unless you continued to use it. Kind of like minoxodyl (sp) does for growing hair (use it, hair grows; stop using it, you hair falls out...).
The stuff did work somewhat to provide surface protection from UV, and would penetrate a little bit into the surface, replacing some of the "plasticizers" in the vinyl and making it a little softer. If you stopped using it it would vaporize out (especially in sun/heat) and leave the material even more prone to cracking than if you had never used it due to the displacement of the original plasticizers.
Both also tended to make the surface slick and a little greasy-feeling.
It would also cause worse window fogging than the original plasticizers (a component of the 'new car smell' of the day) did as they leached out over time, so you ended up with more hard to clean film on your window interiors if your car was subjected to hot sun (or even just heat).
Since they were willing to indicate both ArmorAll _and_ their own product would have the same effect I took them at their word.
The company that started making dash-mats (carpet) in Las Vegas used to include a mylar roll you could trim to fit and put under the mat. They stopped doing so (or so their salesman told me) when they found out the mylar trapped more heat against the dash (under the mat) and caused it to harden prematurely.
I think Carl Walski has indicated the only real preventative.
