We have two main doors where the kids come in at night and out in the morning. There is a lot of milling in that area the closer to dinner time it gets when everyone is jockeying for position to be the first one in the door.
Plus we live at the south end of the Olympic National Rain Forest. No more description needed, right? LOL
We brought in a load of smaller sized hog fuel, a load of sand, then a big load gravel and laid it in a strip several feet out from the doors where the milling happens. Did that 3 years ago now and have only had to add a small load of gravel since. It is pretty hard packed and is only shifting slightly now. They've made other mud bath areas for themselves other places but at least like Mary Lou said, you don't get sucked knee deep into mud.