Carnival Cruise, How do I get a balcony if they are sold out on Carnival?

My hubby and I are wanting to take a Carnival Cruise for 5 days from Galveston, TX during Feb. 2008. I am trying to book the cruise, but I know it is kind of late doing so, and the balcony rooms are all taken. I know that travel agents or discount cruise companies [...]

Carnival Cruise Lines-Look around the ship-8A Balcony cabin

CARNIVAL VALOR® Total Staterooms – 1487— Decks – 13— Passenger capacity – 2974— Total crew – 1180— Tonnage – 110000— Registry – Panama— Ship size – Large (Music:Weezer-Island in the sun.)

How can I convince Carnival Cruise Line to upgrade me to a balcony from an oceanview for free?

I have been on 6 previous carnival cruises, and have never had a balcony and really really want one! Please Help! Thanks!
It is 3 of us, 2 adults, and 1 teenager. Newlyweds idea wont work!!