My vote is for Weldbond white glue. I have a box of about a dozen various glues, and the one I go back to again and again is the weldbond. According to the maker...
Weldbond®  is a universal adhesive that bonds most anything! It is non-toxic, nonflammable, free of fumes, has low VOCs, is created without animal by-products and environmentally friendly!

For my purposes, weldbond dries quickly (but not too fast)to a clear, paintable finish.  Its super strong and water soluble.
Its good stuff, do yourself a favour and pick some up at your local hardware store.

 
As with most excellent ideas on terrain making, this isn`t mine. But I think its a great idea and I wanted to share it with others.
In nature, we see terrain that rolls and breaks up freely, but when you`re building a board thats tough to do with just a carving knife and a piece of foam. An easy way to add gentle rolling contours to your board while still retaining playability is to use `terrain goop`.
The stuff is a mix of pollyfilla cellulose based plaster, latex caulking, glue, vermiculite and latex paint. 
I think the importance of the celluslose plater and the caulking is to provide a medium that is resistant to shrinking as it dries, otherwise you`ll have some corrective work to do. I think this because I just made a batch without the caulking and it shrank up a bit and left cracks. easy to cover with some flocking, but I would have preferred a nicer finish.
Vermiculite is a product you can find in the hardware store in the garden supply section. Its a soil additive that will add texture to the mix.
The glue holds it all together, as you might guess.
As far as mixing proportions go, I dont use a specific formula, just mix it up until it makes a thick paste. I find it stays in place better when its thick, but  if you`re using it a a crack filler a thin mix might be better.
Just smear the stuff around. Instant ground detail. Very nice.