Take a pee bottle - that's a bottle you can pee into in the middle of the night, not an already-full bottle of pee.
Torch, good shoes, cash, lots of cigarettes (if you smoke), more booze than you think you'll need, lots of toilet paper, etc.
You'll walk an awful lot - last year I drunk myself silly over the course of a week and everything I ate was fried (and I ate a lot of it) and somehow managed to lose half a stone.
I put off going to Glastonbury for years, mainly for the aforementioned reasons - a load of middle class kids and old folk being bohemians for a week in a field filled me with dread. Turns out it's only a little bit like that. The rest of it is fairly normal. Devastated I'm not going this year, but have a wedding to attend