Tip: if you want to minimize the blueberries or chocolate chips sinking to the bottom while baking, instead of adding them to the batter, you can try dropping a few on top of each muffin after the batter has been portioned into the liners/muffin tin.
Yogurt: I haven't tried it, but if you don't have Greek yogurt, you could try subbing regular plain yogurt (just make sure it isn't too watery). I've also subbed vanilla Greek yogurt in a pinch; delicious.
Wet Muffins: if the muffins are gummy, wet or aren't baking through, make sure to add baking time and/or consider baking at 350 degrees (instead of 375 degrees) and increasing the baking time so they bake through. Also, using excess blueberries might make the muffins wet/gummy. I use 3/4-1 cup when making this recipe. (This update was added 7/21)