As I mentioned previously, letting the batter rest will help the muffins swell more, encouraging rounded, lifted tops. Also, I set my oven to 425 degrees F for the first five minutes before turning it down to 350 degrees F. This sets the outside of the muffin while the inside is still liquid. Filling the muffin cups a little fuller will help them rise more. Finally, rotating the pan halfway through the baking process will also encourage better lifting.