Pasta and rice will likely be your staples, since they keep for ages and are easy to store and handle; noodles and couscous are likewise, although I don't really like couscous - it's like eating grit. American Long-Grain rice is the easiest to cook, but if you want to use brown rice, pre-cook it for half an hour then treat it as uncooked white rice. Potatoes are heavy but they're worth it. Bread goes off really quickly, but you can just eat it, and you'll probably be able to buy it on larger sites. Try long-life bread if you don't mind the vinegary taste.
Fresh vegetables and fresh meat always taste the best and are the most nutritious. They will last for a weekend, but you'll need to store them carefully to prevent contamination by insects and bacteria. Tupperware boxes are ideal - knotted plastic bags will do for vegetables and cooked meat. Hygiene is especially important outdoors, so make sure you wash and sterilise (with boiling water) all your knifes, plates and containers before handling food. Don't forget to wash your hands
Tinned stuff is easy to deal with - this may be the only time when you'll welcome Spam! You can go a long way with simple things like chicken supreme, corned beef, tuna, new potatos, peas, carrots, mixed beans, baked beans and soups. Tinned fruit and evaporated milk is really nice for pudding! Always retain the liquid from tinned veg because it will be rich in nutrients and salt. Dried, powdered soups and sauces are incredibly convenient since you can carry a lot and they don't weigh very much. Mix soup powder with less water to make an instant pasta sauce. Dried vegetables such as peppers, onions and mushrooms can be added to virtually anything to make it instantly more interesting and nutritious, and bulkier foods like dried mashed potato, soya chunks and soya mince can be the substance of a large meal. You can also enrich a soup or stew with things like stuffing mixture, dried fruit and even muesli.
Most one-pot recipes call for onions fried in fat as a base. They're not essential, but they make a huge difference to the flavour. A small plastic bottle of vegetable or olive oil is perfect. You can also make liquid fat by slowly melting bacon rind. Butter or ghee taste great, but they can be difficult to store in a hot tent!
However you can keep food (and milk) fresh for longer by making an evaporation fridge: put the food in an airtight container, wrap the container in cloth, then immerse it in something like a bucket of water. This has the advantage of keeping the food at a fairly constant temperature however hot it gets - the cloth acts like a wick, keeping a permanent layer of insulation around the container; heat is absorbed into the water which then evaporates, so it'll need to be topped up from time to time, depending on how hot it is outside.
An almost-as-good alternative is to dig a hole - wrap food up in knotted plastic bags to keep insects out, and cover the hole with a board or something. Store food at the north-side of your tent, which will always be the coolest since it never gets direct sunlight.