String: If your clothes get wet, you can erect a makeshift washing line between your tents using string, and dry them out. Genius.
A thick jumper: Even if you're going to a festival in the height of summer (ah, those glorious six hours of British sunshine), you'll still find yourself colder than you thought humanly possibly once night falls. As it turns out, a paper thin layer of canvas isn't the toasty insulator you might have thought. Bring a jumper or six, and prepare to sleep with your whole head inside the sleeping bag.
Flip flops: A terrible footwear choice for the festival itself - you will get trodden on several times an hour - but an essential if you're planning on braving the showers. You will not want to put your precious bare feet onto the shower floor. It'll leave you feeling less clean than when you went in.
A really old phone: Either that, a portable phone charger or a spare phone battery. You will not want to queue for five hours to use the site's one charging point, nor will you want to fork out £10 for thirty minutes of charging. Because with festival electricity, that'll probably only get it up to about 11%.
Jelly: It's rehydrating, and it's a really good hangover sugar rush.
Tin opener: If you're not bringing any canned goods, this one's pretty pointless - but there's nothing more soul-destroying than digging out your tin of peaches for a hungover breakfast and finding you can't get them open.
Eye mask: Not only is a paper-thin canvas not a good insulator, it's also not good at blocking out sunlight. In the summer, when it starts to get light a few hours after you've stumbled to bed, there's nothing worse than being woken up by glaring sunlight at an ungodly hour.
Ear plugs: You know what else tents aren't good at? Blocking out noise. And there's a lot of it at festivals.
Bin bags: You'll accumulate a hell of a lot of rubbish around your tent area, and unless you enjoy living in your own rubbish, a bin bag will come in extremely handy. They're also useful for keeping your stuff dry, and for sitting on if it's been raining and the ground's wet.
Mini trolley: There's always a bigger walk between your car and the campsite than you're expecting, and if you don't have some sort of trolley, you'll quickly regret buying that multipack of Fosters.
Toilet roll: Because there's only usually about an hour a day when the portaloo toilet roll supplies are plentiful, and because there's nothing worse than finding yourself caught short.
Torch: By day two, your phone will probably be out of action (unless you follow the aforementioned advice RE: phones), so there's no use relying on its light to find your tent, or to luminate your toilet trips. A head torch is particularly useful for when you're trying to find your pyjamas and face wipes in your tent at 3am.
Speakers: There's arguably enough musical entertainment at a festival without the need for Spotify, but it's always nice to put on a few songs while you're preparing to head from your tent to the main site. Just don't blast it out really loudly in the early hours of the morning, because that's just a dick move.
Water container: Preferably one that's not solid plastic, for packing ease. We take for granted how much we use taps - for teeth brushing, hand rinsing, face washing, let alone drinking - and having to traipse back and forth between the water source and your tent with just a few small bottles is a recipe for exhaustion.
Wet wipes: If you're not planning on showering, these are what will stop you going from acceptably grimey to full-on disgusting.