Let's cut right to it.
When packing, there are three main points to consider:
1. What luggage should I bring? 2. What are my absolute must-haves (for any trip)? 3. What do I need to bring that's specific to this trip?
Let's start with #1.
What Luggage Should I Bring?
Finding the right luggage for traveling is crucial. You want something that's going to have enough space to fit everything you need, but you also don't want something that will be too heavy and cumbersome for occasions where you're carrying it and/or on the move for significant periods of time.
Most travelers find that they need two kinds of luggage: a larger bag for carrying clothing and gear, and one small daypack for essentials that you want on you at all times. When picking out your main bag, I recommend either going for a backpack between 60 to 80 liters in size, or a duffel bag that can ideally be rolled or easily carried. Because suitcases can be cumbersome to carry, I only recommend them for trips where you likely won't be encountering stairs, periods of time traveling on uneven terrain like gravel or sand, or transportation where a suitcase could be an inconvenience, such as on small boats or crowded public transportation.
When you're picking out your luggage, shop around, and try them out in person. Don't buy something blindly online without having tested it out first. Head to your local outdoors shop and work with a specialist who will find a bag that's not only formfitting and comfortable for you, but also accommodates the style of trip you're planning to embark on.
NOTE: Pay close attention to additional features of a bag, such as its water resistance (does it come with a rain cover, or is the bag itself waterproof?), its durability (how strong is the material, and would it be resistant to elements like thorns, rough handling, etc), and its pockets/storage (are there separate compartments to help organize and carry items such as water bottles, dirty laundry, shoes, trekking poles, etc).
As for picking out a daypack, you want to choose something that has enough room so that it can fit anything you would need on a flight, in transit, and exploring your destination.
Something to consider when looking for a proper daypack is to make sure that has compartments for pens, your wallet, papers and documents (such as your passport), a rain jacket, a small toiletries/first aid kit, and a pocket for a water bottle or two. If you think you'll be carrying a laptop with you on your travels, look for a daypack with an internal laptop pocket. If you're carrying a DSLR camera, look for camera backpacks that can safely hold your camera gear, while still having ample room for necessary documents, transit necessities, and day items.
Typically, daypacks run between 20 and 40 liters, and can also be tested and purchased at your outdoors retail store.
What Are My Absolute Must-Have's?
No matter where you go or what you're doing, 99% of the time, you will have to pack the same necessities. These include:
Toiletries (such as toothbrush, sunscreen, hand sanitizer, and just one small bottle each of toothpaste, shampoo, and soap; remember, you can purchase these things abroad if you run out).
Travel wall adapter. This is for when you're traveling abroad and power outlets are different from your home country's. It's worth investing in a universal converter so you don't have ten different adapters for ten different countries.
Medications & copies of prescriptions. Along with your medications, be sure to bring a broad spectrum antibiotic (which can be given to you from your travel doctor), as well as staples such as Ibuprofen, Tylenol, anti-nausea medication such as Pepto, and Dramamine (if you get motionsick)
Local guidebook (I'm a die-hard Lonely Planet fan)
Headlamp with spare batteries
Reusable water bottle
Sunglasses and glasses (with your prescription, in case you lose your glasses)
Rain cover for your backpack and daypack
Rubber sandals for communal hostel and campsite showers
First Aid Kit
Plastic bags for items that may be wet or muddy
Optional (but recommended):
Headphones (and a spare pair, in case yours break or are lost)
eReader (such as a Kindle)
Notebook & pens
Deck of cards
Insider Tip: Using your shampoo or body wash is a great substitute for washing clothes, so ditch bringing laundry detergent. Pack along a clothes line and pins for drying, too.
What Do I Need to Bring That's Specific to This Trip?
When deciding what clothing, shoes, and gear you should bring on your trip, consider your trip's climate, planned activities, and culture.
The best way to get an idea for what kinds of clothes to bring is by looking through a Lonely Planet guidebook on the region you're traveling to. In every Lonely Planet book, there's always a section dedicated to what clothing is culturally acceptable or unacceptable, what local customs expect in terms of dress, as well as also giving you a month-by-month look at what the climate is generally like in certain regions at certain times of year.
For example: you may be traveling to the Sahara Desert in August, but you'll want to bring a warm sweater and hat because it can drop to freezing temperatures in the desert at night. In another case, you may be traveling to northern Norway in January, which requires a serious amount of extreme cold-weather appropriate clothing. If you're going trekking in Nepal, you'll want a mix of both cold and warm weather clothing, such as hiking shorts and a short-sleeved shirt, as it can get surprisingly hot and humid in the Himalayan trekking regions.
Once you've picked up your guidebook or have turned to Google and you have an idea of what kinds of weather you'll be facing, you'll want to find clothing that's as versatile and culturally appropriate as possible. Specifically for long-term trips where you may be traveling to multiple countries and will have to pack for a variety of climates, you're going to really want to snoop around your outdoors retail store to find clothing that's as lightweight, durable, and compact as possible.
Here are some tips for creating a lightweight and efficient bag:
Bulky fleece jackets should be replaced with synthetic down coats that are designed to not only be extremely warm, but also to pack down small.
Clunky sweatshirts should be replaced with lightweight (but just as warm) alternatives, such as SmartWool pullovers and zip-ups that save space by being form-fitting.
Jeans not only take up space and are heavy, but they're difficult to wash and dry efficiently when on the road. Swap them out for comfortable lightweight travel pants that are durable, multi-purpose and quick-dry.
Dresses and skirts are excellent for hot locations, and or for when you just feel like dressing up. Just be well aware of the cultural expectations regarding dresses and skirts. Do they need to at least cover your knees? Do they need to cover your ankles? Can dressed be spaghetti-strap style, or do you need to cover your shoulders, or your chest, or both? The same rules apply when deciding on bringing shorts and tank-tops. Cultural awareness needs to be acknowledged when packing.
Only bring a handful of underwear and socks. Remember, you can (and should!) do laundry on the go, whether you pay for it to be done, or whether you do while in the shower, in a designated basin at your hostel or campsite. You shouldn't bring a month's worth of these things, as they can take up a significant amount of room, and are easy to clean and quickly dry. I usually bring about seven day's worth of underwear, and wash them as I go.
Find the right shoes. Usually a pair of supportive hiking boots are efficient for most traveling, hiking, and walking you'll be doing, and rubber sandals are good for swimming and showering. Some people opt for bringing along a third pair of every-day shoes, such as a below-ankle walking shoe, which are more comfortable and lightweight for day-to-day walking.
Don't go shirt crazy. It's easy to find yourself wanting to pack a shirt for every occasion, but you'll be surprised with how little you actually need. Instead of bringing your whole wardrobe, pick out six shirts you like, and decide whether they should be mostly short-sleeved or long-sleeved depending on the climate of your destination. Remember that layering is key!
Finally, use organization cubes! Everyone has their own method of organizing their backpacks, but organization cubes help keep luggage from becoming unruly and messy. Especially on long-term trips, knowing where you can find your shirts, your underwear, your medications, and your hiking clothes not only saves time, but also can help save space.
Insider Tip: When women are traveling to more conservative regions, it's always a good idea to bring along a shawl or scarf in case you find yourself in a situation where you need to be considerably covered up (such as visiting a mosque).