Here's things I've picked up:
I always add an extra egg
I add a little Vanilla Extract if I'm making a yellow/butter cake...a little Lemon Extract if I want a lemon-y cake (I generally just buy yellow cake mix and do add-ins to make it taste the way I like it).
You can also add sour cream, applesauce, etc. but I'm afraid to tell you amounts...go to the cake doctor website.