Here are some helpful solutions for the common causes:
Oven too hot
Baking muffins at too high temperatures can cause a hard crust to form. Follow recipe directions for the correct temperature to bake the muffins. To insure the correct temperature each time you bake, always use an oven thermometer.
Muffins baked too long
The best test to determine doneness is to insert a wooden pick into the center of one of the center muffins before removing the pan from the oven. If it comes out clean or with a few moist crumbs clinging to the pick, the muffins are done. Begin testing muffins 5 minutes before the minimum time indicated in recipe.