Have a suitable bike before you make plans. It's amazing how people plan to go on huge journeys, book tickets and so on, before they have a bike. Planning a tour before you have a bike is like planning a wedding before you have proposed to someone. It can take ages to get a touring bike. We seldom have them, and even if you are buying new delivery times vary from weeks to years.
 
If you already have a touring bike, take it to a bike shop a few weeks before you leave to check that it's not about to fail in some disasterous way. We had a Dutchman in with a Rowbike, halfway through a month in Scotland. His front rim had worn through, rendering his machine useless. Naturally it was an odd size, 451mm, but our obsession with alloy rims for Shopper bikes meant that we were able to fix it. It took him a wee while to find us though.
 
Pack everything in plastic bags. There are waterproof panniers, but the prudent will still wrap things up. The rubble sacks sold in big supermarkets and DIY shops are ideal as pannier liners.
 
Have a handlebar bag. These are great for map, camera, gloves, sweeties etc. You can unzip the top and access it while cycling.
 
Don't go too fast. A commuting cyclist will be in a mindset of needing to get there. When touring, relax. You've got all day.
 
Find the simplest solution. We had an american in whose tent leaked. To stop getting wet, he took a small inflatable boat with him as well, put up the tent, put the boat inside, blew it up, and then slept curled up. No. Get a tent that doesn't leak.