Throw several rock clearing parties during the year. I lived in an area that if rocks were an export crop, I'd have been a millionaire several times over. We picked rocks daily at first, then weekly and finally it got to mostly monthly with the stipulation that whoever saw a large rock would bend over and pick it up pronto. The rocks will work to the top of the soil over time, and usually it will work out to where there aren't so many as in the beginning. If you were doing an arena I'd say sand, sand and more sand on top to soften the footing, but not in a paddock where you might feed off the ground.