To expand on what Addie said, mix by hand. Muffins are a quick bread, and quick breads require just enough stirring to get the dry ingredients wet.
Proper procedure is to combine sugar, fat, eggs and any other "wet" ingredient and stir well (you can stir as much as you need in this step). In a separate bowl or in a sifter, combine the dry ingredients and mix or sift. Pour the dry into the wet all at once and stir just until everything is moistened, or as Addie suggested, just fold in. You will have some small lumps, but that is OK, that's how it's supposed to look. At this point the first step of your baking powder is activated and creating small bubbles, that's good, just scoop the batter into the pan and bake.
Over stirring will "pop" that first set of bubbles (baking powder is "double acting" meaning the first reaction is from the liquid added, the second is from the heat of the oven). Over stirring will start to develop the gluten in the four, creating a less tender muffin. Oil makes a more moist muffin, since it is pure fat and butter contains a certain amount of water naturally.
One last thing, make sure the baking powder is FRESH, I would replace it once a year, just before the holidays is a good time since many people will be stocking up on baking supplies then. I use multiple cans a year, so it isn't a problem for me.
