From Arun's comprehensive email to his seniors about how to start the Common Application:
"Read the instructions. In life, always read the instructions."
There's no better way to annoy an admissions committee than to ignore application instructions. Don't decide that you have a better way. Just read–and more importantly, follow–the instructions.
And remember that following the instructions also means not doing things. Don't send more letters of recommendation than requested. Don't include a resume when the college doesn't ask for one. Don't send a recording of your piano recital, a video of your soccer game, or a portfolio of your artwork unless the colleges invite you to do it (which many do, but not most).
Seems simple, but a lot of students get this wrong.