it depends when you go on your second cruise and consecutive cruises. If it is a really popular time for sailing usually use just get room upgrades. If its not so busy at the time you want to go apparently you can get money taken off your cruise. Well this is what my travel agen told me.
My first cruise was jan 2009 and next cruise is this new years eve. So when i booked the TA knew me already and said she would call p and o and see what past passenger discount they could do for me. They said that because it was new years and extremely popular cruise they were unable to take money off the cruise but would upgrade me from the bottom level of the ship ( we chose that cause it was the cheapest and we didnt mind) and now are two levels from the top. At least we wont have as far to walk for everything lol. We were happy with that :)
Im not sure though sorry if u book a cruise and havent yet been on your first.