Considering that nearly every airline now charges for checked luggage (Southwest and JetBlue are the main exceptions on domestic flights), it's a good idea to pack lightly. This isn't always possible, but no matter how long your trip is, pack only four days' worth of clothes. Pick items that can be mixed and matched easily and that can be dressed up or down. Do laundry when necessary — most hotels offer the service, or you can save (and meet locals) by using a Laundromat in town. And if you run out of things to wear, you've got a nice excuse to hit a shopping center and buy a practical souvenir to put to good use.