I'd definitely advocate for traveling with only a carry on, which helps in avoiding the long processing lines you typically wait o while checking a suitcase, and also to avoid the potential hassle of lost luggage. However, we don't always have that option. Packing a carry-on bag is a combination of art and science. Your carry-on bag is the only item guaranteed to get to your final destination with you, so pack the things you'll need during the flight and any essentials that will make dealing with lost luggage easier.
Most airlines allow you to bring one personal item, such as a purse or briefcase, and one carry-on bag. The best way to organize your carry on bag is to separate your needs into three categories: Absolute Needs, Basic Needs and Comfort Needs.
Absolute Needs are, obviously, the things that you absolutely need and could not function without if your luggage was lost in transit. Tickets and medications are two of the most crucial items to keep in your carry-on. This includes prescription medicine or medical supplies, travel documents such as drivers license, passports, boarding papers or tickets and confirmation details for flights and accommodations. A phone charger is also typically a necessity.
For your Basic Needs, it's wise to pack a change of underwear, a simple outfit and something to wear to bed in case your luggage doesn't make it to your destination. Of course you can buy clothing as needed, but having the basics helps tremendously while you are trying to track down your lost suitcases. I also pack travel sized toiletries, such as deodorant, toothpaste, soap, etc, and a small makeup bag for freshening up. Valuables, such as jewelry, money, money orders, electronics and headphones should also be in your carry on to avoid theft.
As for Basic Comforts, knowing that you cannot control the temperature on a plane, and you can't predict whether your flight is going to be overly warm or overly chilly, a jacket or sweatshirt is wise to bring along. A light blanket or pillow is also nice to pack to help you keep comfortable during the flight. In preparation of potential boredom, pack these items for entertainment: a book or magazine, an iPod for listening to music (or for drowning out crying babies or noisy seat-mates) a few light snacks, water and gum.
Consider the length of your flight, and how long you'll be away when packing your carry-on. You most likely won't be served a meal. A few of your favorite snacks will make a long flight more enjoyable. The longer your flight, the more books, magazines and electronic equipment you'll probably need to pass the time. It also helps to be familiar with the security regulations for transporting liquids and potentially hazardous items. With careful planning, you can avoid baggage fees and have a more enjoyable travel experience both in flight and on arrival.