Toss strawberries and rhubarb in a tablespoon of flour before folding into batter to prevent them from sinking to the bottom and releasing excess moisture that makes muffins dense.

Mix wet and dry ingredients until just combined—overmixing develops gluten and creates tough, dense muffins instead of the tender, fluffy texture you're after.

Refrigerate the streusel mixture for 15 minutes before sprinkling onto batter so the cold butter creates larger, more distinct crumbles that stay on top rather than sinking in.

Use buttermilk at room temperature to ensure even mixing and proper chemical reaction with baking soda, resulting in optimal rise and a tender crumb structure.

Insert a toothpick into the center of a muffin—it should have a few moist crumbs, not be completely clean, as this ensures they stay moist and don't overbake.