Like everyone else has said its got to be thick and I think it has to protect them from the elements. Look for areas where natural terrain would block the wind/snow/rain and such. Its got to have many different escape routes incase you come through be in somewhat close proximity to food/water and such. On a farm I hunt IALWAYS find deer beds on a pine tree island in the middle of some lowland.

I've never hunted deer beds, I stick to trails/food and water. G'luck