1. Pack light. I went to at least 6 festivals by bus/train/hitchhiking and I always reconsidered if I needed all the stuff I had to carry on my back. However, this rule doesn't apply to you if you're getting there by car. Actually if you're getting there with a car I would recommend to do the complete opposite - take as many things as it's humanly possible to fit into a car. You never know what you might need.
2. Prepare for any weather. Bring extra clothes and always always always bring something that can keep you warm. You can never know what mother nature has in store for the weekend. There were times when the forecasts were saying it will be sunny and perfect and in real life it rained the whole time. So bring at least one full outfit for any (rainy, cold, sunny, hot) weather. And I just have to emphasize how important it is to bring warm clothes. Even when the days are sunny and boiling hot, the nights still tend to be cold. So if getting sick is not your intention - bring warm sweater(s).
3. Don't let the rain catch you unprepared - take a raincoat or a rain poncho. Yes, I know - they are extremely ugly and uncomfortable and just ew... But it's better to wear one then to get soaking wet and sick the next day.
4. With or without a poncho/coat - enjoy the rain. Dance in the puddles, have a dirt war or create a full body mask from dirt. It's still a festival and don't forget to party in any kind of conditions.
5. This might seem weird but it's very very useful - if you're living in a tent, don't organise your things. Live in total chaos. Yupp, I know it will be very hard to find your things, especially if you're not living in the tent alone, but this tip is coming from experience. A long time ago me and my friends used to keep our things in our backpacks and we would leave them in the tent (because you can't cary everything you bring to the festival). Aaaand we got robed. It's extremely easy for the bad guys to take everything you have if you keep it all in one place/backpack/anything. On the other hand, if there is a mess, no one will find anything (especially if you can't find anything).
6. Bring A LOT of food. If you're a broke university student just like me and can't afford to buy all your meals in the festival restaurants then bring food from your home - it's okay to do this. But just remember to bring two times more food than you think you can eat. Yeah, sometimes it's very hot and I barely eat anything in two days. However, most of the times I feel reaaaaally hungry because of all the extra moving, dancing, partying and being in the fresh air.
7. Don't forget shower wipes/wet wipes. Some festivals have showers, some don't. The ones that do usually have extremely long lines that take 3-4 hours out of your festival experience. If you think that waiting for it is not worth it, then wet wipes are your salvation. I don't think I could ever survive a festival without them. Also, I can wash my hair 2 times a week and they stay tolerable, but if your hair tends to get dirty and greasy quickly, dry shampoo will be your best friend.
8. Don't take your favourite and most expensive clothes. Everybody wants to show off their festival fashion but I recommend you do it in cheaper, disposable clothes (they still can be pretty!). Festivals are messy. Things get ruined, ripped, dirty, lost, etc., so take only the things that you don't mind saying goodbye to.
9. Take an extra phone battery or a portable phone charger. Unless you have a Nokia 3310 that can last for 2 weeks, bring these things! Because your phone will die and you will need to to contact your friends/take pictures for instagram/check facebook/call your mom to tell her you're still alive.
10. And finally don't forget to have a blast! Wether you're going with your best friends or all alone (you can find friends there!) festivals are an amazing experience. It's a place where everyone is 10 times more fun, friendly and awesome. So have a GREAT time! Cheers!