List the essentials. Make a long list of everything you think your family will need. Put the list away for a few hours and when you come back to it, knock off at least ten items you don't need. Remove items that you can purchase inexpensively as well as wishful thinking items, such as books you won't read and socks your kids won't wear.
Plan outfits. You will save a tremendous amount of space (and stress!) by planning one outfit per day, per family member. Items like jeans can be used for multiple days. (Let's face it--a family vacation means you'll be with family. Who cares if you smell?) Save additional space by packing only one or two pajama outfits. Kids can wear the same t-shirt and shorts to bed.
Wear your heaviest shoes. Limit shoes to three pairs, but pack even lighter by trudging around the airport in your clunkers. Don't strain your back by wearing flip flops and carting boots.
Minimize your toiletries. This isn't hard to do with modern-day flight regulations. Spend ten bucks at the store buying mini-shampoos and mini-deodorants. You'll save space, prevent a giant mess and make your kids happy with all the sample-size items.
Bring detergent tablets. Laundry facilities are almost always available. Small tablets are easy to pack and your family can drastically reduce the amount of clothing they bring.
Nix the toys, stuffed animals and media. Allow family members their one favorite item and something to keep them occupied during travel. Leave the rest at home. Stuffed animals take up tons of room, and odd-shaped toys are bulky and annoying.
Substitute quick-dry clothing for heavy coats and jackets. Quick-dry layering is lighter and more efficient than heavy jackets. Remember--you can never know for sure what the weather will be. While it's good to be prepared, bringing something for every weather condition is not necessary. Cheap versions of harsh weather apparel can be purchased.