My number one secret to a great cup-cake is LOTS of icing.
My number two secret is I purposely do not make huge, pop-over cup-cakes. I don't want too much cake ratio to icing.
And ... My third secret is NOT to over bake it. For the last 5 minutes of baking time, place the cup-cake pan directly on top of an air-bake cookie sheet. This allows the middle of each cup-cake to finish baking, while preventing the bottom from over baking.