The key to successful packing is planning. Begin by asking yourself the who, what, where, when, why and how of your trip. Consider the climate, activities, lodgings and transportation involved in your journey. Also consider the people who are going and the purpose of your trip. The next step is to make a list of everything you will need for each person.
A safe bet is to pack one outfit for every day of the trip. This does not mean that you need 7 shirts and 7 pairs of shorts for a 7 day trip. Mix and match clothing to make different outfits and lighter suitcases. When you are on the road nobody else will know you worn those shorts 3 times this week. If you are traveling for more than a week it is worth an hour of your time and a couple of dollars to visit a laundromat and wash your one week's worth of clothing. Bring several big plastic bags for dirty/ wet clothes and shoes.
Remember, when in doubt, leave it out. If you are camping, odds are you will not need a coat and tie. If you will need dressy clothes you should know about it for sure before you leave. Do not pack something on the off chance you might need it. Also, do not duplicate items. One bottle of shampoo and one tube of toothpaste are plenty. If you run out you can always buy more. Over-packing is both frustrating and burdensome.
The best clothes to pack are wrinkle-resistant and wash and wear. They are the easiest to care for by far. Choose bright colors and patterns to hide stains. Avoid black and white as they will show even the smallest specks of dirt. Pack comfortable, all purpose shoes for everyone.
Traveling with a baby will require some additional items. Investigate what crib supplies are available at your destination so you can bring anything else. A stroller is a must for tooling around with a baby and accessories. For toddlers, a baby backpack is a good idea, especially if you plan on doing a lot of walking.
Infants and toddlers need extra protection from the elements so always have a hat, sunscreen and insect repellent on hand. Pack food, formula and diapers lightly and buy more when you get there. However, when traveling abroad, it might be in your best interest to pack extra unless you are certain about what is available in your destination country.
Other items that might be handy are: a battery powered travel alarm (with fresh batteries), a sewing kit, nail clippers, a pocketknife, a flashlight, small packs of tissues, pre-moistened towelettes or baby wipes.
If you will need special equipment for your recreation (bicycles, golf clubs, etc.) see if it is available at your destination. Resorts, especially, have a lot of recreational equipment on hand for guest use or rental. The cost of equipment rental varies, but can be very cheap or very expensive. If rental costs are prohibitive you might be better off bringing your own equipment, but, in many instances, the cost of renting equipment far outweighs the hassle of trying to pack it.
When traveling by car you need only to keep your itinerary, maps and entertainment items with you. You can pull over to get anything else you might need. When you check your luggage on a plane or train you need to take some other things. You must have your tickets/ itinerary with you at all times to check in and to confirm or change your travel plans. It is also a good idea to carry deodorant, toothbrushes, medication and clean underwear on the plane in the event that you are delayed or are separated from your luggage. Commercial carriers are usually good about retrieving lost luggage, but it is terrible to be stuck somewhere without a toothbrush or clean underwear.