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Surround the asters with 2 inches (5 cm) of mulch immediately after planting and every spring.  Before adding new mulch in the spring, remove any old mulch. Mulch keeps the soil cool in the summer and warm in the winter. It also helps to limit and prevent weed growth. Monitor the amount of rain you get each week during the growing season. If you get less than 1 inch (2.5 cm) of rain during one week, you should soak the soil of the planting site.  Asters are sensitive to moisture and will usually become visibly weakened if they receive too much or too little moisture. Plants that receive too little water will usually lose flowers and leaves. Plants that receive too much water may start to yellow and wither. At minimum, you should mix a thin layer of compost into the soil each spring before new growth starts to form. For even better results, mix a balanced, general purpose fertilizer into the soil once a month. Apply the fertilizer as directed on the package label. You'll need to do some light pruning in the spring and some heavy pruning in the fall.  Pinch young shoots in the spring to redirect growth outward. Doing so will create a bushier plant. Cut the entire aster plant back once the foliage dies in the winter. Cut down stem portions that appear sickly or scraggly, or trim the stems back completely to 1 or 2 inches (2.5 or 5 cm) above the soil line. Most aster varieties can withstand either option. Cutting the plant down can improve its long-term growth habit, but doing so will also delay flowering by several weeks. If you live in a colder climate (USDA hardiness zones 5 and below), you may want to wait until spring before doing major pruning. Leaving the plants intact over harsh winters can improve their odds of survival.  You can also remove dead flower blooms on an ongoing basis to improve the overall appearance of the plant, but doing so is not necessary for the health of the plant. If you do deadhead old blooms, do so carefully since new buds tend to be located nearby. . Many asters can grow without staking, but if you have a larger variety that begins drooping, install a stake and train the foliage upright.  The stake you choose should be about 12 inches (30 cm) taller than the current height of the plant. Hammer the stake into the ground roughly 2 to 3 inches (5 to 7.6 cm) away from the main stem of the plant. Use wool yarn or nylon stockings to gently tie the branches of the plant along the height of the stake. Dividing the plant as it gradually becomes fuller will allow it to distribute its resources more effectively. As a result, the plant will remain vigorous and the flowers will remain plentiful.  Wait until spring before dividing plants. Carefully dig out half to two thirds of the established plant. Leave the remainder in its current location.  Divide the portion you dug out into two or more sections. Each clump you divide off should have three to five shoots. These divided portions can be planted in another area of your garden or a friend's garden. Treat these divided portions as new seedlings and transplant them accordingly. Asters don't often have difficulties with pests and diseases, but some varieties can fall victim to powdery mildew, rusts, white smut, leaf spots, stem cankers, aphids, tarsfonemid mites, slugs, nails, and nematodes.  Prevention is better than treatment. Your best option is to choose disease-resistant aster varieties for your garden. When problems do arise, treat them with an appropriate pesticide or fungicide.

Summary:
Cover the area with mulch. Water as needed. Enrich the soil with an appropriate fertilizer. Prune twice a year. Stake tall varieties Divide the plants every two to four years. Watch out for pests and disease.