Mums do not like sites that are exposed to our brutal winter winds. In addition, heavy, wet soils in winter can be another hindrance to overwinter survival. Third, stop fertilizing the plants by the end of July. Fertilizing late in the season often encourages new growth that does not have enough time to acclimate in the fall. Fourth, be a little lazy in the garden in fall and leave the old foliage standing.
Some research suggests that removing the old foliage in fall can reduce cold hardiness of mums.
Lastly, mulch newly planted mums with wood chips, straw or other organic mulches to prevent the plants from lifting out of the soil during the freeze and thaw cycles during the winter. While mums aren't the easiest perennials to grow and overwinter in the garden, they are often worth the effort.