Even though every destination s different, there are a few guiding principles I always follow when packing for an extended trip or an international flight.

1. Know where you're going. If where you are going is the same as where you are, then you should reconsider your destination. Since it likely isn't, make sure you research where you are going and pack accordingly. Going to a jungle? Get bug repellent. Going to the Andes? Take warm clothes. Going to Malaysia? Take cool clothing that isn't too revealing. Make sure your clothing is respectful of local customs and get a feel for whether there are shops where we you can easily purchase essential items if you run out.
2. Buy a Backpack. If you are doing anything other than going from an airport to a hotel and then back to an airport, use a backpack. This seems really basic, but you would be surprised at how many people I have seen on trains, in hostals, even riding boats up jungle rivers with roller bags. A good backpack that is both lightweight and durable is a must have. It will be well worth the cost. Oh, and take your time finding the right size pack that fits you. Name brands don't mean anything if your back and shoulders are sore after only a few hours.
3. Never underestimate how heavy your bag is. You might think it's fine when you put it on in your apartment, but after a few hours of walking, you'll wish you hadn't brought your own bottle of shampoo along. On my first trip to Europe back in college, I brought 3 travel books and a pair of dress shoes in my pack. After a few days I ended up throwing out the shoes and the books.
4. Consider where you are and what you won't be doing. Are you really going to hit a fancy nightclub? You might worry that you will need some slack and leather shoes just in case, but consider reality. If you are traveling you will probably be interacting with other travelers, do you think they will mind if you are wearing casual clothes? If you have a specific event in mind that requires something nice to wear, go ahead and pack it, but if it's just a hope, you could always buy something when you get there.
5. Make sure you check the baggage limits on ALL the legs of your flight. Generally you can have up to 2 bags of 23kg each on internatinal flights going in and out of the U.S. and Canada, but this is rarely the case flying between other countries or on domestic flights. You also need to check with each airline on our itinerary if you change carriers.
6. Plan on returning with more than you left with. This is always the biggest consideration when packing. On one hand I don't want to carry around more than I need to for most of the trip. On the other hand, I need space to bring home all the cool stuff I hope to buy. Solution? I usually travel with a backpack that's bigger than I need and stuff the bottom of the pack with bubble wrap, which is light weight, fills the bag when it's empty and can be used to wrap fragile items you may purchase. Need even more stuff? Buy one of the nylon bags that you put your backpack in to prevent the straps from being caught in airport conveyor belts, they are sold at nearly all outdoor stores (e.g., REI). During your trip it folds up and fits easily in your pack. If you find that you need an extra bag on the return trip, take it out and use it as extra luggage. It's lightweight, takes up nearly no space and is generally very sturdy. I have used those bags for bringing room sized rugs, and even a bedouin tent home.
7. Strategize your clothing. You will likely not need more than 3 outfits, which means 3 pairs of socks, 3 pairs of underwear, 3 pairs of pants and 3 shirts. One of those outfits will be worn on the flight there, leaving you two clean outfits the day you arrive. If you work this out properly and aren't making yourself completely filthy, you can should be able to go about 6 days without needing to do laundry. Anytime you are in the same town for more than 24 hours you can likely find a laundry to clean the other 2 outfits. Whenever I travel I try to pack as little as possible and wash my clothes along the way. If I can't find a laundry nearby, just wash the clothes in the sink and hang them over the heater. If you are traveling to a cold location keep yourself warm by layering, which provides more flexibility over heavy coats and jackets.
8. Only take what you need. This seems like an easy one, but it's easy to let go. Things like prescription medications you should take, things like aerosol deoderant can stay home. If you can buy it there, you probably don't need to pack it unless you will use it everyday (e.g., a toothbrush).

Hope some of these ideas are useful. Got anymore packing tips? Share them in a comment.