Overbaking or using too much flour can result in dry muffins. Be sure to stir, spoon and level the flour when measuring into cups and don't overmix the batter. You can also pull the muffins from the oven when they are just barely underbaked (when a toothpick inserted in the center comes out with just a few moist crumbs), as the residual heat from within the muffins will continue to bake some.