PalenQ has the right idea about using the automated machines. Once you use one 1 time, they are pretty easy. Just make sure you wait for your ticket and receipt to drop.

As for the train time differences, I'm surprised no one has brought up the train differences. The main trains are ES, IC and IC+. Look in the trains section in Trenitalia to see the differences. IC+ is slower but cheaper. IC+ also has all new trains and no smoking. Some ES trains were looking pretty tired last year.

When looking at a fare on Trenitalia, click on details and then on the train number, which is highlighted. This will tell you if you have any stops or not and where they will be.

If your trip date is more than 90 days out, wait until you get within the 90 days and print out the page, in case you decide to go earlier or later.