Here is a suggestion as soon as you get on the ship have the tips removed from your sign and sail card. I prefer to give the room steward $10 on the first day and let him know immediately that you would like to always have ice in your room and a specific amount of glasses. Throughout the cruise I make sure I give my steward tips here and there, even leaving a thank you card 2 or 3 times on a 5 day cruise just as appreciation.
We prefer to tip according to the service we receive, not prepay or should I say tip according to the service you hope to receive.
During the Celebration cruise in August 2006 our service was good at first then went bad and then worse. We met a couple on the ship that had a steward that never provided ice in the bucket and didn't clean the room until after 1PM one day. Then removed the tip from the sign and sail card on day 3.