I mix wall putty (the one in powder), white glue, various grains sand, a bit of soil from the garden,cat litter and water. No problems of shrinking or cracking, dires reasonably quick (and rock hard) and takes/wheels tracks prins very well, and it's virually unexpensive...
Ciao

Someone already mentioned Woodland Senics' plaster cloth--and it's the BOMB! But here's what I've come up with after only two weeks at this game: Figure out the exact size of the base. Then get it cut out of 3/4" plywood or (other solid wood)--you don't have to get fancy--it just has to be thick enough not to warp.
Sand all sides, top, and bottom until smooth. Again--nothing fancy--it's going to get covered with stuff!
Next put on a layer of acrylic gesso (get at art supply stores). One or two coats sanded lightly in between will do it. After it's dry, it's ready to take on the under terrain material.
I used plaster cloth over some terrain that I built up following some ideas I found in books about modeling model railroad scenery (bunched up newspaper held in place with masking tape, aluminum foil, etc.). This depends on whether you need hills or other high ground. If not you can make the lumps 'n bumps with the usual sand, rocks, kitty litter,--or even bulit up layers of the plaster cloth and extra plaster or acrylic modeling paste (you can add color to all of these--I'm an artist so I have plenty of acrylic paint laying around that I've used with great results).
After you get your base & terrain all done & it's dry, you can add more paint for a top coat to match your ground cover layers. I mixed the acrylic paint with acrylic matte medium to keep the colors dull.
When you are done, you can insert this into a nice frame or attach finished moulding around the outside. This method is fairly cheap, easy--and pretty spiffy looking if I do say so myself.
Let me know if anyone one else tries this & what results you had.
Have fun!
--daredevil [email][email protected]]

I always use Polyfilla Ive never had to use glue with it.I just mix it up with some sharpe sand dried coffee grounds then I mix up some cheap craft acrylics to the rough ground color I want then put that into the mixture then spread stops any white spots,I found it never shrinks or cracks,I just finished the base for my sherman that was a piece of styrene packing from a furniture box I just sealed it with some Unibond and spread on my usual mixture also it is easy to dig a nice shell crater out of it. all the best.

I use 70% celuclay, 30% plaster and mix in some white glue. The detrious is kitty litter and fine sand. The base itself is mdf and machines well. It can be painted to llok like anything from wood to metal.