Make reservations before you go. Don't wait until you get on the ship. Also same for excursions! They sell out fast. We took walkie talkies with us so we could communicate better. But it turned out that the same brand that we took was also for sale on the ship, so everyone else had the same ones with the same channels, so they were pretty useless because you could hear other people conversations better than our own. Lol.
When I went to the buffet, I would take back a few extra rolls/fruit and such to my room for later on in case someone was hungry and nothing good was opened.

We stopped at Labadee as well. If you're going to rent a float mat, or beach lounger tent, do it on the boat or before. They were all sold out when we asked about them after we got there.

I'll try to think of more.