If you have enough clothing for three days you have enough for a short trip. A longer trip means washing and re-wearing the clothes. But you still have to factor in any specialized items. Take sporting gear and swimwear only if you are going to use it, and if hiring isn’t feasible. Take something that will get you into a formal venue, even if you aren’t planning on such an event.
Use clothes that don’t need ironing. If you don’t have a good jacket or other formal-wear try rolling them into tube to prevent wrinkles. Some good quality jackets, perhaps of courser weave, resist wrinkling more than others.
Buy some cheap socks for the trip. Discount stores sell packets of socks that are cheap and dry quickly. If they are all identical you don’t have to worry about matching pairs. These socks aren’t long lasting, but they will survive the trip. Put them, in a clothing bin at the end of the trip.
Compression socks for a long aeroplane trip are a good idea to prevent aching legs. You’ll only use them twice per trip, but it is worth it.
If you can go disposable with any item consider doing so. Toothbrushes, a cheap watch, towels, umbrellas, older t-shirts and other items can be recycled at the end of the trip and don’t matter if lost. If you accumulate souvenirs like most of us do, then disposing of some items will give you some much needed extra room.
Three pairs of shoes covers most situations; pack more only if you have a specific need for hiking, sailing or specialized shoes …etc. a good pair of comfortable shoes is essential, and a more formal pair. Thongs or sandals are useful for the beach or showering; these can be disposable. Pack shoes in plastic bags, and store items inside (like socks) to save room. Always wear shoes that are at least a little broken in.
Either get luggage that can separate item into individual compartments or put various types of items into sealable plastic bags. Have a plastic bag for socks, underwear, shirts …etc. This keeps things organised and is easier if you are searched at customs. Plastic bags also keep items dry in case of a downpour. Take a large durable plastic bag for laundry.
Buy electrical adapters for foreign countries, they are cheap on eBay. But remember, they only change the socket connections, not the voltage. Some items tolerate this, many don’t.
Consider renting a mobile phone in a foreign country. Using you own phone can prove horribly expensive, check with the provider. You may not need to talk with locals on the phone when overseas, but an internet connection may prove essential; local maps and information is almost always on the net. Keep in contact with family by email.
Wear something confinable for the journey overseas, remembering that weather conditions can change abruptly. This is especially important if travelling to the northern hemisphere where the seasons are reversed. Sunglasses, a hat, a jacket, sunscreen …etc. should be part of your carry-on luggage.
Earplugs, an allergy free pillowcase, a small torch, noise cancelling headphones, a portable clothesline, an iPod, and other items can be useful to some people.
Passports, tickets and other items need to be safe and on your person when boarding a plane. Make provision for this.