I put about 2 inches of pea gravel and 3 inches of sand in my run (over dirt). I got advice from another byc'er that has used this method with good success. I only just finished my coop a couple days ago so I can't tell you how well it works!
The person who suggested it said that all you need to do is rake it a couple times a week and the gravel and sand sort of pulverize the poop and it sifts down. They also recommended adding new sand and gravel as needed, I assume a couple times per year...but we'll see...
good luck!