Don't pack too many formal shoes! Try to make sure each pair goes with at least two dresses. If there are only two formal nights, I never take more than one pair.
Eye mask and ear plugs if you are disturbed by light or noise.
Small bills to tip for room service.
Don't overpack especially if you are flying. I always being a bit of washing powder to wash some smalls that I don't want to send to the laundry. Your bathroom will have a drying line.
Find out how many and which type of plug points your ship has. Very important for charging all those devices! I also bring my kindle.
Check your travel insurance covers cruises.
Warn your bank that you will be at a different port everyday so they don't block your card thinking it's been stolen! This happened to me.
Don't forget sunscreen, sanitary products, deodorant or any essential bath products! I also bring some basic medications eg paracetamol, ibuprofen and anti sickness tablets. The ship doctors are expensive to see.
Most of all: ENJOY! 