The number one rule for successful muffins is: Don’t over-mix the batter.
Use a rubber spatula to quickly fold (gently stir) the liquid ingredients into the dry ones. A few streaks of flour still showing is okay, especially if you plan to add in extra ingredients like fruits and nuts. Do that with a few strokes – five or six should do it.Check the expiration date on your baking powder. You might be surprised how long you’ve had it. Unlike baking soda, baking powder can get too old to do its job of making your muffins rise up. You can test whether your baking powder is still good by proofing it. Toss the expired stuff and get fresh.
Save the paper liners for cupcakes. Some muffins will stick to the paper. Papers will also keep the sides of the muffins from browning nicely. Use cooking spray on the muffin tin instead.
Spray the cups well if your muffin batter is full of fruits as these can get sticky while baking and adhere to the bottom and sides of the cups.
For ease and size consistency, use an ice cream scoop if you have one to fill the muffin cups.