There are only two ways to get an upgrade.
1. you have paid guaranteed and its possible you might get an upgrade but are guaranteed to at least get a cabin at the level you booked.
2. You pay extra money to upgrade further up the ship or to a higher level cabin. This one is the only one you can ask for and definitely recieve.
Life is too short to let the ship of your dreams sail without you.
Carnival Destiny Feb. 2006
Carnival Fascination Feb. 2007
Royal Caribbean Grandeur of the Seas Feb. 2008 The DTW & MsBJ tour
Carnival Valor MsBJ and Dina Feb. 2009
Carnival Glory MsBJ and Dina Feb. 2010
Carnival Dream MsBJ and Dina Feb. 2011
There is 2 other ways to get an upgrade. If the price of your cruise goes down, you need to call them and either ask for ship board credit or an upgrade.
The second way is to ask for a handicap room. These rooms are almost as big as a mini suite. Even if you are not handicap, they will give it to you under the warning that if someone really needs it, they will reassign you. However, many times when they reassign you, it's an upgrade from your category!