Make plans to beat the heat. Tennessee is notorious for sweltering, humid summers. Plan for the heat in advance and bring plenty of water. Think about how much you'd normally use in a weekend and triple that. Also take portable fans and a tent with the maximum ventilation possible. Bring along a tarp as well. It helps to shade your tent. Bonnaroo is held on a cleared farmland with almost no natural shade. Bring as many coolers as you have room for in your tent also.
Be prepared for any medical mishaps. Carry a fully stocked first aid kit with you to administer to any cuts and scrapes that may occur. Bring various medicines for common ailments like headaches and upset stomachs. Be sure to bring sunscreen in a high PH and zinc as well. Being prepared for medical mishaps also includes bringing insurance cards too.
Dress for success. The weather in Tennessee changes rapidly, especially in June. Pack for a variety of weather situations. Bring a bathing suit and light cover-ups, airy summer clothes and a rain coat or slicker as well. Don't forget to pack some variety in your shoes too. You might be able to get away with flip-flops or sandals all weekend but if the rains muddy things, you'll need boots or galoshes to make it across the fields. You will also need a hat to protect from sun and rain.
Pack appropriate camping gear. Make sure the tent will accommodate every one. You should also bring a large screened tent to serve as a dining or living room area. You will need somewhere to spend your downtime. Pack a sleeping bag and pillow as well as any gear you will need for cooking. Don't forget the chairs and something for light such as a lantern and flashlights.
Pack food that will not easily spoil. Think along the lines of canned foods and non-refrigerated items in bags and boxes. Snack food like chips and pretzels make great Bonnaroo food. If you plan to cook all your meals bring classic picnic fare like hot dogs and lunch meats. Don't forget utensils like forks, spoons, knives, plates, bowls and glasses.
Do not forget the miscellaneous little things that really make or break the trip. Be sure to pack several rolls of toilet paper. Don't forget the towels and wash clothes, soap and hairbrush, either. You may also want to bring small cameras and phones.