Are you camping at it? If so you won't get much sleep all weekend because people will stay up all night being rowdy and they have big floodlights in the campsites so it constantly seems bright outside. I remember on the Saturday at Oxegen I was woken up by SOME MESSING and I thought "Shit, I wonder what time it is, band's might have started!" and then I checked and it was actually only 4am or something. That's all part of the fun though, I felt, it didn't bother me.
Make sure to bring wellingtons too because if it rains the ground turns to slush. This is just
the tip of the iceberg! (http://www.youtube.com/watch?v=FATeNhfPvCI) Coming back from Muse on the first night I thought I was going to cry because it was so hard to actually lift your legs and the route from the arena back to the camp was one that I hadn't travelled before so for a long time I didn't even know where I was. I bumped into one of the guys who I went with though and we WENT BACK TOGETHER.
Another thing you might want to do is bring a few rolls of toilet paper and anytime you have to go to the toilet take them with you because they don't get restocked at all often. Baby wipes are good too for cleaning mud and stuff off.
Food and drink is ridiculously overpriced too, so keep that in mind and have plenty of money and preferably some stuff of your own so you can avoid having to stump up the money for all the crazy expensive stuff.
All in all man, I reckon you should have a great time just so long as you don't have OCD or something because the facilities definitely leave a lot to be desired.