When we first started cruising we brought everything, just incase. Now with each cruise we bring less and less...we're now down to one very small carry-on and one suitcase each for a 10 day+ cruise. Our clothes are all the wrinkle free, light weight type, etc.
When we're going to cruise Alaska for example, we wear our jackets so we don't have to pack them. Same with heavier type shoes, etc. If you can wear it, you don't have to pack it.
On one cruise to Alaska, my sister and brother-in-law traveled with us and besides their luggage she brought a TRUNK...we had to have a seperate cab just to hold their luggage. She couldn't decide what to take so she brought everything they MIGHT need. They were shocked at how little luggage we had and we were shocked at how much they had!
The only MUST HAVE'S for us, have been...our travel documents, Personal I.D and Medications. Other than those items, if we happen to forget something, we've found we can easily buy it on or off the ship.
Try this: Just before you finally close your suitcase, take OUT one or two of those iffy items.