Depends on the cruise line, usually if you put at least a deposit down you will get some on board credit on your future cruise. For instance if you put $100 down on a celebrity transatlantic you get $300 credit(longer cruises get more). Most of them don't require you to actually book on board just put a deposit down and you can pick the cruise later.
I was on a cruise and inquired about staying on for the next sailing and was offered a "special price" and was told not to tell anyone. Apparently there was a sale going on later that week only for Florida residents(which I'm not), but she said no one would check.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp