After your plants have been killed back by hard frost, cut them back to the ground and provide a light airy mulch of evergreen branches or similar material. The most dangerous hazards for Chrysanthemums in the winter are constant wetness or ice on the leaves.
Frost heaving caused by alternate freezing and thawing of the soil can also a major problem. To help with these problems and keep the plants somewhat protected and dry, you can mound the soil up around each plant, as well as creating a simple drainage ditch alongside.
To help avoid the heaving try to keep the plants somewhat shaded and insulate their roots with a light airy mulch.