A couple years back I rode from Boston to Yellowstone, Tetons, Rockies and back... 7000 miles in 12 days.
Rain gear, rain gear, rain gear... wicking layers, fleece layer, dry socks and gloves, foot powder for feet and nether regions, snacks, powdered Gatorade to add to water bottles (10% dilution works great), battery charger for phone, 1/2 height bike cover, spare glasses, tire pressure gauge and 12V mini pump if you have a Tender or 12V outlet, chain lube, ear plugs, good tires, an itinerary left with others, maps, gps, camera, spare batteries and a SPOT if you have one to let others track you.
Most importantly a positive mental attitude and awareness. Soak up the trip like a sponge ad try to enjoy every mile. Stop and take pictures when the mood strikes... it adds interest and forces you to take a break.
Have fun and ride safe!