Pinching promotes numerous flowers and reduces the need for staking. For fall-planted mums flowering takes precedence over root growth and establishment. The second best time for mum planting is late August to early September to allow at least some time for establishment before cold weather sets in. Don't wait until December to plant your mums. Mums have a shallow, fibrous root system so water thoroughly during hot, dry weather.
Mums also respond well to additional fertilizer during the growing season.
After the ground freezes trim off the dead stems and place them back on top of the plant crown. Mulch with shredded leaves for additional winter protection. If you buy mums this fall, look for plants in bud with only a little color showing. Plant as soon as possible and keep soil moist but not soggy. The color dresses up a garden so much I don't care if they become fall annuals.