When it comes to touring, there are very few truths with which everyone can agree, but lots of opinions. That said, Pat did provide advice regarding flats and tires that are likely common to most, and John expanded on the maintenance items you may wish to consider.
What follows are other thoughts you may find helpful as you plan your trip. These are just thoughts as you will decide what works best for you.
I ride on Continental Gator Hardshells, 28's. I tried Schwalbe but did not like the feel (felt like I was pedaling through molasses).
I do not ride ultralight, but I do like to minimize weight while trying to stay comfortable (similar to backpacking), so I do not mind paying for weight-savings. Again, similar to backpacking, an ounce saved here and there can make a difference in the ride. Toward that end, at one time I traveled with the two person tent I used for backpacking. Now I have switched to a one person tent, Copper Spur by Big Agnes. With the footprint, it packs small and weighs less than three pounds. I am six feet tall and weigh 180 to 185, so compared to a two person tent it is not terribly roomy, but it works for me and I enjoy not having the extra weight.
I also have a Big Agnes Lost Ranger sleeping bag, with a sleeve into which I use a neoair sleeping pad. Again, backpacking gear.
I generally carry four water bottles. I ran out of water on one of my earlier trips when my planned water stop had a sign that said the water was not potable. I did not enjoy the next twenty some miles without water. Hence, I travel with four, which is likely overkill but is helpful when I have had to dry camp.
I take three sets of bike clothes (wear one, pack two). When I first started touring I had two sets. For me, three works better.
Consider wet weather gear, depending on where your trip takes you. I have spent a full day riding in rain and having rain gear that kept me somewhat dry made the day less trying.
I do not take any cooking gear (I do not want the weight). I generally eat at restaurants, when available, and I do carry a number of bars for when restaurants may not be available. I also carry two Mountain House granola breakfasts. Add cold water and you have a meal that provides you with needed nutrition. Again, this is for those days when I may not find a restaurant. On one of my trips I stayed at a rest area that was miles from anywhere. Those bars and the granola breakfast made that doable.
I now carry a Kindle Paperwhite for reading. The battery last several weeks before I need to recharge.
I try to limit the amount of electronics I take. Again, weight. And frankly, the reason I like riding is for the solitude. I do have a cell phone and I take a Samsung Galaxy Tab that is not dependent on WiFi (I started with a 3G, now I use a 4G).
I vacillate between taking a foldable tire or not. The last two trips I have not taken a foldable tire but I do take a kevlar tire boot. Again, on one of my earlier trips the nut holding the rear rack failed, which led to the rack falling and shredding about six inches of the rear tire, exposing the tube. I was more than 50 miles from a bike shop that had my size tire. It was a challenging two days. So now I carry a tire boot or a foldable tire.
I carry four tubes. Yes, it is overkill but all overkill has a story behind it.
I only have one pair of shoes, and those are the shoes in which I travel. They are MTB shoes, with frog clips, so I can walk normally. I do take flip flops. My earlier tours were with pedal cages. I now like the frogs.
I take extra nuts and bolts used on the bike and the racks and I carry a couple of zip ties.
I now tour with front and rear fenders. When roads are wet and someone is following you, they will appreciate that you have fenders. Hell, your front and back will appreciate that you have fenders on wet roads.
I carry bike maps for each trip. I like maps. I mail home maps I no longer need.
Depending on where I am traveling, I will take TP and a latrine trowel (backpacking supplies) when I find myself camping somewhere that may not have facilities.
Okay, that is enough. Enjoy the ride.