Most of this may be old information for you multi-tour veterans, but maybe it'll help the person getting ready for that first tour. My tour took me from Seattle to my home in Louisville, KY; a total of 3075 miles. I used the Northern Tier maps for about 2/3 of the ride.
1) Shorts - Buy new shorts for the tour if your's are worn. My old shorts were fine for 2 hour local rides, but the padding didn't hold up. I finally bought new shorts about 600 miles before the end. What a difference.
2) Maps- I had the ACA maps and they were great. However, once I left the route, it was hard to find a decent map that showed county roads. Gas stations don't carry maps. So be sure to pack a map for each of the states you'll pass through. I know, lots of people have smart phones or PCs, but there's nothing better than a printed map.
3) WiFi - Its almost everywhere. I paid extra for a Verizon mobile hotspot and web account. It wasn't worth the extra money I paid.
4) Food - In the Western states, there aren't a lot of grocery stores. I hate carrying lots of food around, but at a minimum keep one day's supply in your bag.
5) Tires and tubes -The roads in the Western states are hard on tires and tubes. There is more gravel and it seems to be sharper. Many times I'd dig out small rocks from my tires. I had two flats and they were in the first 1000 miles. Once I got into MN, WI, MI, etc. I had no problems. Check your tires for imbedded rock chips, especially before a long descent.
6) Chain Cleaner - When the roads get wet, the bits of gravel and sand will stick to your chain. Have some system/product for keeping your chain clean. (Most local riders don't start rides in the rain, but when you are touring you'd better be prepared to ride in the rain. Otherwise, you may not move for days.)
7) Money - I kept my wallet well secured. Each day I'd take out enough cash for expected expenses and put it where it was easy to access.
Camera - Convenience is more important than features. Take an easy to use, light weight camera that you can pull out and use at moments notice. You'll take more snapshots than photographs.
9) Miles - I averaged 75 miles per day, but had as few as 32 (headwind and rain) and as many as 130 (tailwind and net downhill). If you need to stay on a timetable, keep riding when the riding is good.
10) Heat, Fluids, Food - In the hot weather, its easy to drink lots of fluids, but as everyone says "drink before you are thirsty." If you can sweat and keep moving, you can remain remarkably cool. Hot weather diminishes your appetite, but make it a point to eat good healthy meals. If you slow down or get tired, its not so much the heat as the lack of fuel. I had 100+ mile days when the temp was in the 90's.
I hope this helps someone planning their first tour.