Good article. We're fortunate in having soil that drains well. We also have interconnected runs that are pretty large (for nearly 40 birds). One is totally denuded, but the two others are full of weeds -- and that's where the birds like to hang out, in the cool shade and shielded from raptors.
To build up the soil (and provide the birds with some fun), I've raked huge piles of leaves into the bare run, dumped mulch hay there, or even dumped old littler from their coop, the latter containing lots of wood shavings. Going the sand/gravel route, while a great idea, would be expensive for us because of the amount of material required.