There's a lot out there written about packing for long trips, not least because it's a lot more difficult to do right than is packing for short trips. Nonetheless, here are a few techniques that I've found help me to pack well for long trips. Note here that I frequent urban areas, so you may need a different mix for traveling to rural destinations.

• Pack blends, not cotton. The more polyester, the faster the clothing will dry -- which is good, because you will be washing all of your clothes by hand. How many changes of clothes should you pack?
• Count the days -- that is, the days between clothes-drying opportunities. In most cases, your blends will dry overnight if you squeeze the water out and hang them to dry in your bathroom. How many times will you not have a whole night in your hotel room to let your clothes dry? If you'll be on a train, plane, bus, overnight hike, etc., you should bring an extra change of clothes so that you have something to wear while your clothes dry. Then bring an extra change of clothes for a margin of safety. I've easily made it through a 30-day trip with just 3 changes of clothes using this technique.
• Pack dish soap. To wash your clothes with, silly -- it works well and a little goes a long way, which means you can pack a small container and easily make it through your trip.
• Emergency items -- duct tape, extra shoelaces, stuff like that -- just takes up space. Unless you really expect something to fall apart (perhaps you have my old, rickety frame pack that fell apart regularly), you probably won't need these things, and you can buy them easily.
• After you've packed, get your next smallest suitcase and re-pack all of your stuff in the smaller suitcase. You'll rapidly discover that you aren't really all that attached to the things that don't fit. You can either take the smaller suitcase and be more mobile, or move everything back to the larger suitcase and have room for your gifts.
• Again, no cotton (or wool). This especially means you, denim jeans. Jeans are great but they'll never dry if you wash them, so pack some synthetic pants. In particular, I like:
• Zip-off convertible pants, which can have the lower legs removed to convert into shorts. The best kind can be zipped at two spots, so you have long shorts, with which you can get into most cultural and religious sites, as well as long pants and convenient, cool shorts.
• Socks. This is one thing to pack extra -- never wear wet socks, and never wear uncomfortable socks. Socks (and shoes -- but you really only need one good pair) are the key to any long trip.
• Plastic bags are your friend. Put most non-clothes items in plastic bags of their own to avoid trouble from spills; bring extra bags for wet clothes or the tchotckes you find.
• Copies of everything. Xerox your passport, driver's license, and traveler's checks, and hide the copies safely in your suitcase (in the lining, if you can manage -- if you have a rolling bag then it's often possible to get an envelope of documents past the handle's supports).
• Sufficient toiletries. It's often hard to get the brands you're used to in other countries (in many cases, the products of the same name in another country will be appreciably different). If you're comfortable trying something new, then pack light; otherwise, eschew the travel-sized items for the smaller, normal, packages you find in better-stocked pharmacies.
• There are packing folders that can be used very effectively to carry one or two more-formal shirts and a pair of formal pants, without worrying about wrinkling.
• Pack books. English-language selections can be poor in many countries, especially outside of the largest cities. Books can be left behind and thus your bag gets lighter as your trip continues.
• In terms of packing mechanics, start with the formal clothes and then add the toiletries and books; finish by packing your rolled-up socks, underwear, and t-shirts around the spaces. Wrap your belt around the very perimiter of the bag, it takes too much space up if you fold it over itself.
• If you'll be living out of your suitcase, rather than having a chance to unpack, then you may profit from packing your bag in "zones" -- an area of clothes, an area of toiletries, and leave an external pocket for dirty clothes. Within the clothes "zone", pack each day's outfit in layers, such that removing one day's outfit reveals the next layer. If you put your fancy clothes in a packing folder, pack that at the top because you can easily take it out and put it back in.
• In today's regime of limited carry-on bags, it doesn't make sense to try and fit a change of clothes and your toiletries in your carry-on. Instead, make sure that your carry-on is comfortably set-up as a daypack -- that way, you can tour even on travel days, rather than being weighed down by all the clothes that fill your pack. A daypack should contain:
  • A camera or camcorder
  • Your guidebook
  • A rain shell or umbrella, and, if it will be cold, a fleece jacket
  • Water
  • A snack (a Clif bar or the like is a good thing to have around in an emergency)
  • Band-aids
  • Toilet paper
  • Passport and money
• In general, if you're not sure you'll need it, don't bring it! It's a long trip and you don't want to be schlepping the whole time.