This image is fairly easy to reproduce. The key is having a terrain like this in the first place, with all this erosion.
But then, secondly, it depends a bit on whether you want to make a close-up and distant shot, like in your example image.
For a distant shot this is relatively easy. You can use a regular surface shader, coverage near 1, and a pretty high max slope contraint. That should be it.
Alternatively you can play with the intersect underlying feature to give preference for the snow to primarily cover the depressions (your eroded slopes). Ususally you need a second surface layer to also cover a bit of the rises (so intersect underlying switched off on this second surface layer). Intersect underlying sometimes requires another compute normal shader before the surface, you will have to experiment here.
Technically, that's all you need.
You can also have a look at this thread:
http://forums.planetside.co.uk/index.php?topic=7152.0 for another example render similar to yours.
Lastly, if you are up to make perfect close up snow patches, take a look at this NWDA preset here:
http://www.nwdanet.com/buy-packs/details/25/7/preset-packs/tu-snow-packHere is one other example rendered using the TU Snow Pack:
http://forums.planetside.co.uk/index.php?topic=7072.0Regards,
Frank