John,
I use heavy white felt from the fabric store as a base. You can use this on upper levels too, or use the cotton snow blankets. The problem with those is that they are not very wide, so you may need several rows of it. Then just let it drape over your upper level platform and cut to shape as needed to meet the main level. For the upper level I use plywood with 2x6 on edge for supports, which give 5" of clearance below if you want to run trains under the platform. Also get the fiber fill material which you can use for some small hills or under the fabric/mat to give it some shape. You can see my village in run 315. Another tip, if the rubber brick streets don't lay flat due to being rolled up, hit it with a hair dryer.
Don
