1) Too much flour can make muffins tough. Always measure flour carefully by spooning the flour out of the storage container into a dry measuring cup. Level the top with a flat edge. Don't tap or shake the cup to settle the flour because the flour becomes compacted and you'll end up using too much. 2) Stirring the muffin batter too vigorously can also cause muffins to become tough. Use a light hand when mixing the dry ingredients into the wet ingredients. Stop stirring as soon as the flour is incorporated. It's okay if the batter looks a little lumpy.