I've only been on two cruises; a three-day on RC and a 10 day on the QM2. I gave each cabin steward a twenty dollar bill upon meeting them. A seasoned cruiser told me that she'd had great luck doing this.
Service seemed good but I didn't ask for anything special and am rather inexperienced. I would certainly like to see others who have done this post here.
If I pre-tip and the service is poor, I would simply reduce or eliminate any tip at the end of the trip.
Any other thoughts?