Shoot in manual, you'll need to ramp the ISO up quite often, it can be very gloomy. You'll be mostly around the 100-200mm region in my experience (I go every year). Take a wider lens too, some of the corners can put you quite close to the action where even the 100 end will be too long.
Non photography tips; arrive the evening before, by 4am the entrances to the stages are parked up and you'll end up with a long yomp to get on stage. The atmosphere in the evenings is great fun, sleep in the car rather than in a tent, often there is no-where to pitch a tent. Bring wet weather gear, it can be ghastly up there, although the dates this year will place it a bit better than the usual November / December timing. Take wellies or walking boots, the paths can get really torn up, particularly if its raining, which it usually is. Bring some drinks and snacks with you, the queues for the overpriced grot vans can get big between stage running.
As Callum says make sure you're safe, listen to the marshalls, they WILL stop the stage if people don't move when told.
I hope this helps and enjoy.