Decent and broken in walking boots, forget wellies, rain gear - trousers and jacket, you'll look like a gobshite but be nice and dry underneath, can be quickly removed if rain stops or heading into tent for a gig.
Earplugs, blindfold, camp bed or decent self inflating mat, decent hiking boots, foldable water container (holds a gallon so only one trip to the taps each day, if it gets too hot, hey presto - a shower), BREATHABLE waterproof jacket - forget about the plastic ponchos you will just end up soaked in your own sweat, shorts, socks and jocks, t-shirts, a decent hoodie, sunglasses, sun block, micro fibre towel (they dry in no time if laid flat), multipack of toothbrushes in case you drop one, TP/Baby wipes, beer, buckfast, spirits along with a backpack for same, and a decent camping trolley to put it all in - Never get taken to the cleaners by boutique camping again!
...seeing as I'm now mid 30's unfortunately 5-HTP, Berocca, Dioralyte and pain medication are now also required!