General principle 1: take as little as possible
You have to carry it. The more I travel, the less I take. How much will you actually use? How many changes of clothes? Depends on you, but most archaeological projects last so long that you will have to wash things anyway. I'd have site clothes and off-site clothes, and perhaps an emergency spare set, but you can get by with relatively few clothes if you take things that wash and dry quickly. Modern fabrics have improved incredibly for this. Try some of the cotton/polyester or cotton/nylon mixtures used in "travel shirts" and "travel trousers". They dry in no time, pretty much wrinkle free. For very hot climates, however, you may be more comfortable in pure cotton and, if it's that hot, drying your washing really isn't a problem. The bulky towel problem has similarly been solved by compact synthetic fabric "travel towels" - a medium sized one will dry you all over (you wring it out between wipes) and still dry out in a half an hour or so.
General principle 2: don't take anything you really mind about losing
Don't have the best quality, most expensive anything. No personal items you care about. All your stuff will get hard wear, be used by other people, and could be lost, or stolen. You may just want to throw away some of your clothes at the end of a dig. Excavations are generally not good places for sensitive hi-tech equipment. Laptop computers fare particularly badly, what with dust, fluctuating power supplies, damp, coffee and worse. You'll almost certainly want a reasonable camera and the same problems apply. In such cases, all anyone can do is make sure it's properly insured, do the best you can to look after it and then stop worrying too much.
I have a vast array of rucksacks, bags and cases accumulated over the years. A good rucksack (expensive!) is a requirement if you will be carrying everything with you and walking a good deal, but they are not ideal on long overseas trips. Any soft bag fares badly strapped to the roof of a taxi/bus, or on the conveyor belts of an airport, and rucksacks are a particular problem because the straps stick out. Also, fabric bags are a security problem. You can padlock the zip but someone with a razor can easily get in, and I know people this has happened to. If you are excavating abroad, the chances are you will be travelling about in cars, buses and taxis so there are often few advantages in a rucksack. For camping I find a zipped kitbag (with plastic bag inside) best, because you can easily fit it in the tent, but for overseas projects I mostly use a medium size hard plastic case with wheels. I can just get a few clothes, sleeping bag, mosquito net etc. and all my dig gear into it. It has survived numerous airline and other assaults, and has many scars to prove it. I can sit on it when there's a long wait, and I can lock it with its combination lock (small luggage keys are about the most loseable thing I can think of). All the old advice applies. Don't put your home address on an outside label - do you really want that strange person to know where you live? And these days, don't ever leave your luggage alone or with anyone you don't know. Pack well inside your main luggage anything that anyone might for any reason at all think could be a weapon, and that includes much basic archaeological equipment such as trowel and penknife.