Mistake #1/On-Tour Training
Some people figure they can get in shape on the first week of a road tour. They find out pretty quickly that they should have done it earlier.
The best approach, if you haven't been riding much, is to start training 2-3 months beforehand. Start by riding 15-20 miles a day, 3 days/week. Raise your mileage 10% per week. The slow, gradual approach is always best.
Look at your trip itinerary and find your longest day. If it's 90 miles you should be able to do 70 miles training. You can ride about twice as far in a day as you've ever ridden while training. But do some long training days to see how your body reacts.
Mistake #2/Ignoring the Mental
Be mentally prepared. A lot of people quit a tour because they let things get to them. It's not a physical problem. You have to be prepared to face difficult times and work through them. You have to convince yourself that you'll keep going.
Mistake #3/Not Getting Loaded
If you're going on a camping trip, do at least 2 training rides with the bike fully loaded, especially if you've never ridden with panniers. The extra weight changes your bike into a different beast, stability- and handling- wise.
You have to brake earlier because your bike's heavier. And you have to stay on top of the bike more in turns. If you lean the bike too low, your front panniers can hit the ground. Also be careful on rough roads, because the initial shock on your wheels is greater with the added weight.
Mistake #4/Neglecting Butt and Hands
Get a comfortable saddle, even if it's heavy. Chamois salve works for some people, but baby powder is great, especially at night to keep yourself dry. Sometimes, though, there's nothing to do but work through a miserable break-in period. It usually lasts about 3-4 days. If you'll be renting a bike, take your own saddle.
As for hands, padded gloves are great, but moving your hands around the handlebar is the key. A drop handlebar is good, but a mountain bike with bar ends or an aero bar works too.
Mistake #5/Riding the Wrong Bike
A hybrid is still the best, although more mountain bike-style frames are being seen as well.Touring bikes are good. A 3-chainring setup is a necessity. With 2 rings, you just can't relax on the climbs - you have to stand. Smooth-tread tires are a must too.