One big reason for the cracking is varying rates of expansion; as the top layer hardens, it no longer expands and contracts at the same rate as the bottom layers, which in some cars can be a steel or aluminum substrate or stamped framework. This is especially evident at openings and corners, where the need for flexibility to accomodate expansion is greater. (the expansion is "photographic" where everything gets bigger, including the inner edge of an opening, not just the outer edge of the dash)

Aside from applying your solution of choice to keep the dash pliable, minimizing the expansion and contraction will also help a lot: use an opaque sunshade as much as possible, even for indirect sunlight.

The car's glass absorbs short wavelength UV (UVB), which helps a lot, but passes the long wavelength UV (UVA), and of course, much of the visible light, which greatly heats up the dash and raises temps, leading to cracks. I'd almost say that heat (and the expansion and dryness it causes) are more of a dash-killer than UV itself, but what do I know? I haven't even stayed at a Holiday Inn yet.

Good luck, and stay flexible.