Be sure to measure your flour correctly using the spoon-and-level method. Fluff the flour in its container, spoon it into a measuring cup, and level it off with a knife. Scooping directly from the container can pack in too much flour, leading to dry and dense muffins.
If adding ingredients like chocolate chips or walnuts, fold the batter until it's nearly combined, leaving a few pockets of flour, then gently mix in the add-ins until just incorporated.
Important Note about Salt: Not all salts are made equal and cannot be substituted 1:1. If you are substituting table salt for kosher salt, you will need to cut the amount called for in the recipe by at least half.