think in advance!
Clothing/Shoes/Weather Gear: The problem with rock festivals in countries like Germany is that you will probably get wet because of the rain. A light raincoat is ideal (I have a small one that can be fold up and have it handy in my small back pack). Only a few people hold umprellas (pic 1). The second problem is that even a passing shower will turn the whole area into a mud area. This means that you may need an extra pair of shoes (some people prefer boots). I try to have something comfortable because after 3 days of standing you wont feel your feet from the pain if you wear boots. Some people wear trousers with huge pockets to hold many cans of beers! (pic 2)
Toiletries and Medical Supplies: The first time (2002) I forgot the sunblock so I returned back home with a red nose. Now, I always bring sunblock with me (and a hat of course). There are places with showers in some parts of the camping area. I put my tent somewhere close so to have a shower every morning but I also have a pack of wet baby wipers with me at my tent that are very helpful (you have to believe me)
Photo Equipment: Video cameras arent allowed inside the stage site.
Camping/Beach/Outdoor Gear: A good tent is a must, in Greece we use some simple ones but a 3 season tent is better for Germany because of the rain and higher levels of humidity during the night. A small flashlight is always useful when you search something in the dark. I noticed people (germans) that have gas stoves and things like that but they use a car to carry these things.
Clothing/Shoes/Weather Gear: High heels are never a good idea (especially if it's muddy). A variety of clothes is helpful, because weather forecasts can be wrong, best to be prepared for either a heat-wave or torrential rain (one of those raincoats that folds up very small is a great investment, trust me!).
Toiletries and Medical Supplies: Sunblock is a good idea should the weather be good. It's also a good idea, if you're at a camping festival with limited shower access, to bring a pack of wet wipes or baby wipes. They're great for sponge bathing.
Camping/Beach/Outdoor Gear: Obviously bring a tent if you are staying on the camp-site, and if you can carry it, air mattresses are great as the ground isn't always level (or stone-free). At Wacken, unlike other festivals I have been to, it doesn't seem to matter what you take onto the camp-site, so a gas stove and camping saucepans could be useful, as you can buy food in the supermarkets in town.