I love Glastonbury. I've managed to trim my packing to the following after fifteen years of festival going:
Cut off jeans, 2 pairs saw me through last year's deluge - pockets are dead handy, you can tart up your look with your top
Couple of pairs of tights to wear under cut offs if it's chilly/muddy. So much easier to dry off and smaller to pack
Army surplus poncho - easier than a jacket, water rolls off, you can put your bag under it. Roll up and shove in rucksack if under cover and use as something to sit on, easier to negotiate the loos.
Hat with brim - better than a hood and can be used in rain or sun (I've got a waterproof lightweight Fjallraven one)
Wellies, even if it looks dry, do not go without them - loos flood etc. Better they sit in your tent than you don't have them
Knee high socks if wearing wellies without tights helps prevent wellyrub
Battered old comfy trainers if dry
Couple of fab tops - depends how many days you're going, I went for 5, took 3, bought a t-shirt there, didn't care about wearing same one twice
Comfy, warm hoodie - clear skies can mean a real chill. I took a thin but super warm merino one which sat in my daypack most of the day
I took a merino buff and beanie for this reason - even when boiling in day, was glad of it at night
My non-negotiable - extra bra and clean knickers for each day: I don't mind wearing muddy/dusty outer layer as long as I've got clean knickers
Festival essential: I took a completely over the top vintage rose gold sequin jacket/cape thing which I threw over everything and it made me feel instantly fab and ready to dance - was £7 on eBay so I didn't mind trashing it (I didn't, as it happened!)
Couple pairs warm, non cotton socks (cotton rubs when damp from mud or sweaty heat!)
Sunglasses
Bare minimum/maximum impact make-up: black eyeliner, full on lippy, glitteryshit - use up the ends of your make up bag or buy eg Rimmel
Wetwipes - I take smaller handy packs
Hand gel
Travel dry shampoo
Don't take loo roll, take tissue packets and stuff one in jeans pocket at all times- less likely to drop/get damp. Home Bargains do cheap big multipacks
Headache stuff
Diarrhoea stuff
One of those tiny concentrated bottle of squash things to add to water when you're dehydrated and don't fancy just warm water
Empty fabric conditioner bottle for early morning tent wees - wider bottle neck, empty in loo later on (don't pull a face, you will not want to trudge 100 yards for a wee at 3am just after you've got into your sleeping bag. Trust me, thank me later)
Couple of eg Trek protein bars for emergency energy/breakfast when you've over done it in the sun)
Headtorch - so much easier than carrying one
Roll of black bags - sit on, emergency poncho, emergency bag up your stuff if tent leaks, So good for so many things
Don't take anything you're not happy to drop in mud, down longdrop, have stolen or lose.
If you can be arsed, a camping chair makes SO.MUCH.DIFFERENCE to a comfy, chilled festival experience - we usually pick two for fifteen quid ones up on the way at the services
Wine in a box
Decant your nice booze into plastic bottles, buy chilled mixer there
Hip flask
Platypus water bottle
Little day pack - to chuck essentials and booze in, if damp/rainy, put your stuff in black bag first to line/keep dry
Leave a dry & clean clothing set in your car boot with clean shoes to change into before driving home with wetwipes, tissues, bottle of water and snacks in case of long wait in car park.
God,I love festivals.