Involve the kids
Get the kids on board by involving them in the planning. Their specific role will depend on their age and your patience, but it could be colouring a map of the route, reading books or watching movies about the destination, helping to find a hotel within a budget, and planning which attractions to visit. They can also participate in the packing. Even small children can choose a favourite toy and book to pop in their suitcase (although it doesn't hurt for you to pop a few extras in your suitcase, especially if you suspect that your child has made a choice he or she will later regret).
Keep them busy
Books, toys, stickers, games, portable DVD and game players, books on tape, and music CDs to sing along with can all make a huge difference to how happy your child is while en route. You could ever put together an activity bag for each child, perhaps keeping it as a surprise for when they get into the car or onto the plane. Once you're at your destination, find a way for them to blow off some steam by playing active games or going for a walk on the beach. Physical activity is a great stress reliever for the whole family.