Preheat the oven: Don't bake before your oven reaches the proper temp. If you're cooking at lower temperatures, muffins won't reach their full, peak potential. High temperatures, about 375º to 400º, will give muffins lovely, peaked domes.
Check baking powder expiration dates: Old powder prevents batter from rising correctly.
Have all ingredients at room temperature for best blending. For example, ice-cold milk will harden melted butter and keep it from blending evenly.
Don't overmix the batter—it's okay if it's a little lumpy when it goes into the muffin liners. Stirring flour and liquids too much can create tunnels in the batter, which makes the final product tough. Add wet ingredients to dry, mixing just until moistened.