First off, make a packing list. Preferably one for each member of your party so that you don't overlook something (I do my own luggage, DW has enough to keep her busy). There are tons of packing lists online, almost all of which are very similar. I downloaded several, combined them into one list, edited it for duplicates and removed stuff I'd never take, and then added the few things I wanted to take but were not on the list (for example, Disney magnets for the cabin door, lanyards and trading pins, etc).
I also do a day-by-day chart of some kind showing each day's planned activities (excursions, theme nights, beach & pool, etc) and any special clothing or items I might need for that day. Then I then add up how many shirts, pants, shorts, etc that I'll need for the week.
Now comes the hard part - cutting out about 1/2 to 2/3 of the clothing. I try to wear pants, shorts and shirts at least twice, or maybe even three times on a cruise. Most times these aren't worn all day but for just a few hours at a time (especially if you're spending time at the beach or pool), so no need for several outfits each day. I have two outfits I wear for dinner, alternating them each night. We dress up for formal night and I wear the same outfit (my tux) for semi-formal night as well to justify the weight and space. DW takes an LBD and just changes the accessories. Shoes are tough, but I try to keep it down to just 3 pair.
Kids are a totally different issue and what you take is heavily dependent on the child. Some kids need multiple outfits every day (so plan on spending some time in the ship's laundromat), while others may be able to get by like an adult. For small children, pack outfits into individual zip-lock bags so all you need to do is grab one and dress them, and then you can put the dirty clothes back into the bag.
I don't take anything extra for "contingencies." I just make do. I have a light windbreaker that I take, which serves as my jacket to the airport, as a rain jacket, and for any cool days that may occur on the cruise (we had really cold temps one year at
Castaway Cay).
About a week before the cruise I do a practice pack: I pack everything I plan to take and check it off on my packing list. This is a good way to (1) make sure you have everything, and (2) that it will all fit in your luggage without busting the zipper or giving the porter a hernia. If you have too much, get rid of some of it. Also, for a family consider cross-packing, putting at least one outfit for each person in everyone's luggage so in case a bag is delayed (or lost), everyone has a change of clothes. The practice pack should also include your passports, cruise documents, etc. After you finish, you can unpack whatever you may need before the final pack. When you do the final pack, use your packing list again so you don't forget anything.
If you don't have a travel luggage scale get one! I weigh my bags at home before I travel, and then when I repack to go home. I try to keep my bags about 5kg (10-12lbs) under the weight limit when leaving home so I have weight (and space) for souvenirs.
When the cruise is over, go over your packing list, add anything you wish you'd taken, and note anything that wasn't needed. For the next cruise think twice about packing it what you didn't need. This may not apply to all things: I have a small flashlight in case of a power failure or to help me get around the cabin in the early morning without disturbing DW. I don't always need it, but I never leave home without it. I've reviewed my packing list after every cruise (and other trips as well), and now it's very rare for me to add or delete anything.
Bottom line: Don't wait until the last minute to begin packing, or pack without a list. You'll end up forgetting something important like someone's clothes, your clothes, passports, etc. It happens a lot.