Ever notice how snow in soft light tends to sparkle? This is from the fine ice crystals in the snow reacting to light, these crystals reflect bright light when the light strikes them from a particular angle, and since you have thousands of fine particles that are all at various angles to the light and camera at any given point, little points of light "blink" on and off dependent on the angle between you, the snow, and the light illuminating it. When all three of these angles and distances remain the same, there's no sparkling effect.

The best way to simulate this of course is to have a specific programmed shader do all these calculations for you, but through expressions you can create a similar effect, an effect which may work in several simpler circumstances. The material is a snow material I've had for awhile, the surface has a wide and tall highlight, and noise in the specular slot. The noise is heavily clamped, achieving little dots of white on a black surface. This noise is then placed in a mask, and the mask also has a similar type of noise, the idea being that though certain "holes" in the mask you're able to see the little pinpricks of white in the first noise. And if you animate the phase of these holes based on the distance you're viewing the surface from, you'll get that sparkling effect you want. To find the distance between the surface and the camera, I wrote a simple expression, read up more on expressions in the max manual, and see the example file below to see the expression implemented.

Here's a zip file containing the original max file (This file requires Worley Bump, part of Digimation's Essential Textures from Worley) and here's a max4 version of the file that uses regular everyday noise instead of worley bump. Render out the animation to see the sparkling effect. Note that this is just a quick experiment, since the sparkling effect is also dependent on light distance and angle, you may wish to try customizing the file with a more complex expression. And of course, for all those of you who live north and you have tons of snow on your porch, here's an excellent opportunity to do even further research.