Tip #1: Use room temperature eggs, butter, and Greek yogurt. Generally speaking, cold ingredients don't mix as well together as room temperature ones do.
Tip #2: Use clear imitation vanilla extract. Not only is it cheaper than vanilla extract, it also creates a more authentic Funfetti flavor. It keeps your muffins whiter (given that vanilla extract is brown) and retains more flavor than vanilla extract does when baked at higher temperatures.
Tip #3: After combining wet and dry ingredients, gently fold them together with a spatula. This will make it less likely that you'll over-mix your batter.
Over-mixing can create a dense, less fluffy muffin.
Tip #4: Bake at 425°F for 5 minutes, then, keeping the muffins in the oven, lower the temperature to 375°F. Continue baking for 12-13 more minutes. This helps them rise and create a nice, domed top.