What about the Heat and Sun?
The sun can be a problem on fieldwork even (rarely I admit) during the British summer. You might be used to sunbathing, but hard work under a merciless sun all day is a completely different proposition. There is usually no shade on an archaeological site and the heat and light are reflected back into your face from the trowelled surfaces and drawing paper. It's difficult to provide effective sunshades that don't interfere with the work. So you need to keep the sun off with your clothes. Of course, if you start to feel unwell then you should get into some proper shade until you feel better. People who are unused to the sun need to take particular care for the first week or two, but even people who have lived all their lives in hot countries can be affected. You need to pace yourself a bit, and the rhythm of life is a lot slower in hot places for very good reasons. In particular, everyone on site needs to drink lots of water. You will be amazed by how much you can drink working hard in the sun. That's a whole big bottle for a morning's work. You will need a hat with a brim large enough to shade both your eyes and the back of your head. You will also need efficient sunglasses (proper UV protection British Standard etc.), because the glare can be pretty intense. Don't take expensive sunglasses, as they have a hard life on site (lost, buried, sat on, abraded by blown sand). I manage to destroy several pairs a year, and usually take spares, but find that a hard case improves their survival. If it's really, really hot, you need to be completely covered up and pure cotton clothes are more comfortable than fabrics with synthetics in them. People often come back from digging in places like Egypt completely untanned, because the sun is just too hot. A long-sleeved shirt with a collar is more suitable than a T-shirt, because you can cover your sunburned forearms with the sleeves, or your neck by turning up the collar. The neck is particularly vulnerable because you will be continually bending over to work (classic "archaeologist's neck" sunburn). Short trousers are cooler and more comfortable than long but, if your legs start to burn, you will need to cover them up. Loose and baggy long trousers are a practical solution, or you could try those trousers that convert to shorts by zipping off the lower part of the leg. In addition to all of this, a good quality, high-factor sun cream or lotion is essential. Use it on your face, hands and any other exposed bits even if you are otherwise well covered up. Sometimes the sun looks hazy doesn't seem too bright, but you can still burn. Lips get dry in the heat and dust too, so lipsalve is a good idea. And while you're at it, the dust dries out your hands and can make them sore, so a little hand cream of some kind can be a good thing.
Outside Europe or North America, it is important to take local dress customs into account so you should check on this before you pack. In many Islamic countries, for example, men just don't wear shorts and women are expected to be reasonably well covered too. If you ignore this, it's unfair on locals who might be working on the site and may well lead to difficult situations.