While this topic strays a bit from our usual IT management fodder, I’m frequently asked for packing tips since I spend an ungodly amount of time in airplanes.
Perhaps the best tip I’ve found is the most simple: lay out what you think you will need, and then put half of it back in the closet, especially for a longer trip. Bring flexible clothing that you can layer, or that serves multiple purposes (for example a comfortable “upscale” pair of shoes you can use for evenings out on the town yet also wear for long walks around an unfamiliar city).
I also recommend creating a reusable packing list (this could be as simple as a document on your computer that you print for every trip). When you get home, remove the items you didn’t use from the list and after several trips you will have the “perfect” packing list customized for your needs. I categorize mine for groups of items that I may or may not need depending on the type of trip, so I have a group of items for international trips, items for running if I’ll have time for a run on a trip, and cold weather items.
Finally, nearly every town in the world has a dry cleaner with a “wash and fold” service that is priced quite reasonably. Most hotels also have this service, but it can be quite expensive except for many hotels in Asia. For long trips, this can really cut down on your packing, since you can stop in once or twice during the trip and have undergarments and whatnot washed and folded. I even do this occasionally before heading home so I don’t have a “gift” of a suitcase full of laundry for my wife!