Then after tossing your berries gently with some flour (this helps them to not sink to the bottom of the muffin), gently fold them into the batter with a spoon. (kneadtocook.com)
Mix about a tablespoon of flour with the raisins, so that the raisins will not sink to the bottom of the muffins. (killerbunniesinc.com)
Tip: To keep the blueberries from sinking to the bottom of the muffin, toss them in a bowl of flour to coat. (humanesociety.org)