Lots of clean socks.
Put on a fresh pair before going to sleep each night.
( Edd: The Unfunny Try-Hard Edgelord, Tue 16 Jun 2009, 9:53, archived)

And I'd recommend keeping you valuables (phone & wallet etc) on you at all times.
At other festivals, I slept in a sleeping bag fully clothed with the items in my pockets. Thieves are less likely to root through your pockets to risk waking you up.

Also, on the first night, try and stay up as late as possible. Thieves won't go near you if you're still up and the first night is always the one they prefer to strike since people will have more money on them then.
( Edd: The Unfunny Try-Hard Edgelord, Tue 16 Jun 2009, 9:56, archived)

Well now i can see why people go to such things, i believe the newest Glastonbury advert slogan is:
Give us cash and we'll ensure you'll fear for your belongings and health.

Also: Some bands.
( Mykeyboy A massive, salty filing cabinet, Tue 16 Jun 2009, 9:59, archived)

It's just being sensible.
I've never had anything stolen from me in the 5/6 years I've been going. I've once had someone try to get into my tent but I was awake and he quickly ran off.
( Edd: The Unfunny Try-Hard Edgelord, Tue 16 Jun 2009, 10:02, archived)

Hitch up one of those attack alarms to the zip
But get a custom one to play the car alarm from Phoenix Nights.
( Maximinimus you stick around I'll make it worth your while, Tue 16 Jun 2009, 10:12, archived)