You get your total bill the last night or you can check your account from the TV in your room. You can then go to the Guest Servies desk and put a cash payment on your account. You can put cash payments on your account at any time during your cruise.
__________________ Carnival Valor May 11 - 18, 2014
Ruby Princess - Nov 11-18, 2012
Monarch Of the Seas - Oct 18-22, 2010
Radiance Of the Seas - Oct 14-19, 2009
Sovereign Of the Seas - Sept 26-29, 2008
Explorer Of the Seas - May 15-22, 2005
Voyager Of the Seas - May 19-26, 2002
On past cruises as we put any purchase on our account we would go and pay it with travelers checks. About every other day including the day before we return. That way we would now how much we had left. Any last minute charges were just left on our account and paid by pre authorized credit card. That way we were able to avoid the long lines on the last day. Note that this was on Princes and Disney, we are booked on the Navigator in December 2005 and I will assume that the process will be similar.
With all the different cards out there with "rewards" for their use, we prefer to charge everything and then pay off the card each month. It does take will power though (and cards with no fee or an acceptable fee) but we get the benefits along with never paying interest on the card(s).