1. Everyone who has ever done bike touring will tell you to limit your possessions to an absolute minimum, taking only what you need, getting your weight as low as possible. And for goodness sake you should listen to them. Everything you have is weight and volume that could otherwise be spent on something better or better yet not be there and not be slowing you down. People with support and gear vehicles (SAG) have a so much easier time because they don't have the weight of their own stuff. So, really, think very carefully about what you need and you don't. Ask yourself: do I eat it, drink it, sleep on or in it, maintain my bike with it, or maintain me with it? If not, you don't want it.
2. Thoughts on specific things in regard to weight- Always carry water and convenient food, but remember you only need so much. Know the distance between you and water, refill when you can, and you'll be good without needing some big water container. You probably don't need a bunch of survival gear, of course depending on the where, when, and what of your trip. In rural areas, a heavy bike lock is just the needless weight of mistrust. Your phone can be GPS, camera, books, blogging device, entertainment, and basically all electronic needs in a super lightweight package.
3. In the deliberation between a trailer and panniers, I honestly can't tell you what's better. But I can say both work. Trailers can carry more and the weight isn't on your bike, but they make maneuvering trickier, take up more space on the road (which can really matter on narrow shoulders, especially narrow with rumble strips), and I think make for more work and trouble with stopping and acceleration. If you do use a trailer as I did, solid tires seem the way to go. Worrying about 4 tires is just too many, small tires are a huge pain to get on and off, and solids on the trailer aren't something you feel too much. It just seems the better way to go. I can't weigh in about solids on your bike.
4. Carry your bike over all grass. Never trust it. Thorns will ruin your day, and puncture more effectively than almost anything. Just don't risk getting them.
5. On a related note, forget slime inner tubes. They're just more expense, weight, and mess, and all they seem to do at best is let you know where the leak was.
6. I put on a patch over a puncture that lasted over mountains and for more than 500 miles. It did eventually wear out. But it's just something to keep in mind.
7. Dear lord, if your tire tread is looking worn, replace it immediately. Carrying a spare tire might even be worth it. And of course always have inner tubes, at least two at all times.
8. A hydration pack you can drink from while biking is hugely helpful. I couldn't have imagined the trip without mine. And no matter what judgments you might get (which you really won't), a belt bag or something you can put accessible food in is really good too. You don't want to stop all the time you have to eat and drink. You'll lose precious momentum and rhythm.
9. You will need to eat a lot. You will probably need to eat like some ravenous creature you don't recognize. Food is fuel. Don't ever compromise on getting what you need. And quality matters alongside quantity. A good, healthy breakfast can make all the difference in a day.
10. If you do some planning beforehand, Warmshowers and Couchsurfing can make your life much, much easier having places to stay. Also, people on Warmshowers seem to generally be more understanding and suited to your needs since they're specifically there for cyclists. But that's not to say the people on Couchsurfing aren't equally awesome. They're both great resources to use to keep you from tenting every night.
11. If you are tenting, city parks are great to use wherever they're legal (say in Nebraska). If you have to pay, it's always cheaper to go with the small local places. Screw KOAs.
12. I know some cyclists are big into guerrilla or stealth camping, staying someplace out of sight and mind without explicit legal right to, and it generally sounds like it works okay (except in the South, everyone universally says it can get ugly there), but I think legal, and free, options are usually available and better. Knock on a door and ask to camp in their yard, try out a church, just generally ask around, and someone will probably be happy to help. Permission is a great thing.
13. In any situation, don't be afraid to ask for help. Especially when people see you're bike touring, they are usually more than happy to assist.
14. ...Except on the interstate. If you break down there, expect it to take an hour of flagging for anyone to stop and help. In fact, in general, assume everyone is less good when in a vehicle. That said, interstate isn't such a bad place when you can be on it, because you've usually got a wide shoulder, which matters far more than how stupid fast the traffic is going (if a car hits you at 60 or 75, you're equally dead).
15. A road bike or touring bike is probably the way to go. I used a mountain bike because it's what I'm used to and because you can get a decent mountain bike cheaper than a decent road bike. And it really did come in handy for some stretches of trail. But most the time it will just be slowing you down. Take all the help you can get from good and proper equipment that you can. But also know that you can go over a mountain on a cheap mountain bike hauling a converted baby trailer full of not the lightest gear. You may hate yourself for it, but you can do it.
16. Remember as you climb that mountain, you are accruing potential energy karma. And what goes up must cone down. Eventually you will get to reap that karma, and it will make it all feel worth it.
17. High headwinds are seriously the worst thing you will face. Unlike a mountain, they just take and take and give nothing back. They're maddening and demoralizing. Going west is harder than east in the U.S. because we're in the Westerlies wind belt and so the winds will be against you more than not. If you have 30-40 mph winds on a day, be kind to yourself and plan on going nowhere if you can.
18. Actual, tight fitting breathable bike clothes are totally worth it. I only had one pair and I wished that I could trade my other clothes in for them. I also had four sets of clothes and three would have probably been better.
19. Solar chargeable battery for your smartphone is incredibly useful. Find a way to mount it so it gets sun all day and it will serve you well between outlets. Also public bathroom outlets are your friend. Keep your phone charged at every opportunity and as much as possible.
20. Just remember to have fun. Yes, it will suck at times, there will be boredom and hardship, and you'll probably mess some things up. But you'll learn most by your own personal mistakes and experiences. Roll with it, trust the road to provide, and just keep pedaling through. Something amazing is always just past the scary road, through the rain, or up the mountain. After all, it's the contrast, the punctuation of struggle and reward, that makes it all so worth it. Wonder awaits just around the bend.