The more you mix a batter, the stronger the gluten in it becomes, and the tougher your muffin will be. Here are a few tips:
- Create a well in the dry ingredients in which to pour to wet ingredients. This makes it easier for the two to combine.
- Stop mixing when no streaks of flour remain. If you are going to fold in blueberries (or nuts, chocolate chips, etc.), stop mixing when a few streaks remain. Folding in the add-ins will take care of the rest of the flour.