The ONE tip: pace yourself. Your body is not used to walking 15+ miles in a day, sleeping on the ground, and partying until sunrise. Day 1 energy is a trap — start slow, build up.
Bring a portable phone charger. Not a small one — the biggest one you can find. 20,000mAh minimum. Your phone will die by 2pm on day one otherwise.
Make friends with your camping neighbors immediately. These are the people who will watch your tent, lend you a phone charger, and become your festival family for the weekend.
A short dispatch each week. Lineup announcements, new festivals, and quiet gems worth the trip.