From this thread.

I've been to several festivals before and really it boils down to making sure you bring what you need to be able to relax and have an enjoyable time whilst at your campsite.

Have plenty of food and water is the first thing - you don't want to have to purchase food or ethanol at the festival as it will be expensive. Make sure you plan out each meal that you will need and make sure that your perishable foodstuffs will be kept cool for the duration of the festival.

Bring a medium sized basin to wash your dishes in. You can get galvanised basins at the hardware store that you can fill up at the public fountains. Bring metal flatware and plastic plates that can be washed and dried easily. Don't bother with paper plates - they are a waste and are annoying, especially if you are trying to reduce the refuse that you will inevitably produce.

Bring several plastic trash bags and crush your aluminium beer cans before disposing of them. If you are not conveniently close to a public fountain, make sure to have a large (>3 litre) bottle to store your day's water each day. If you need extra water for dishes and washing your feet, fill that at the same time. For soap, the best is Dr Bronner's Hemp Oil Soap. You can use this for washing your hands and feet (and hair if you have access to showers) as well as washing your teeth, your dishes, and whatever else needs cleaning. It is a universal soap. It's perfect for camping.

If you are going to have a portable gas grill, make sure to have plenty of fuel since fires are not allowed.

Keep all your precious belongings inside your tent whilst you are at the stages. You can get locks for your tent but thieves are less likely to steal your things if they are out of sight. If you are going to put your cooler in your tent (as I advise you to do) make very sure it doesn't have a slow leak that will waterlog the inside. As a safety precaution, a small tarp and towel underneath the cooler will help in case.

Have plenty of towels, kerchiefs, and serviettes handy. Some for the dishes, some for your hands and face, some for your body, and a few extra for good measure. You cannot have too many of these.

A canopy to have over your camping area is a must. You will want to be away from the damaging sun and it is very hard to do that without just being in your tent. You might even be able to make a makeshift one from tarp and poles, but unless you are a decent structural engineer, it runs the risk of flying up during a heavy wind. (I actually saw an unanchored tent go ten metres in the air during a gust a few years back.)

Wear baggy short trousers with large pockets on the sides. You will want these so that you can carry all your belongings in closable pockets. It also helps to be able to bring a few beers close to the venue and if you're lucky you might be able to sneak a few in. The large pockets will allow you to bring a bottle of water, your camera and car keys, and maybe even some small snacks without having to schlep them around in your hands.

Hope this helps. Have a good time, enjoy the shows, and stay hydrated and sunburn free. Cheers!