Here's what I took for camping at the Festival Internacional de Benicassím 07 (F.I.B.)...
light zip top for chilly evenings
other shoes for the campsite (ground is a bit rocky)
toilet roll - essential!
face wipes - essential for an instant fresh-up :)
basic first aid kit (plus First Aid is also available on campsite)
sheets/material to create shade/blankets (you will be truly grateful of the shade!!)
hammock - much more comfortable, cool and free than tents in the heat
dry foods - crisps, biscuits etc
phone + charger (can charge mobiles at the campsite bar)
head-torch - essential at night!