17 cruises (and counting!) and we had a problem with debarkation only once--the time we made our own air arrangements! After years of letting others handle stuff for us, I guess we just weren't as savvy as we thought and hadn't fully considered the amount of time we'd need to leave to get to the airport. We screeched up to gate about five minutes after they closed the plane doors.! Fortunately we were able to get seats on the following flight so it turned out okay. The cruise lines do a great job of collecting, sorting, and delivering baggage and getting folks off the boat in a timely fashion. Keep a good book, crossword puzzle, or cards handy to help you through the wait. Most lines debark passengers needing wheelchair and, those with priority (early flights, club status) first.
The only "don't" I can think of is "Don't sit on the steps or on the floor!" I've seen folks do this even when there are plenty of lounge seats that are far more comfortable.