How early should I make train ticket and reservations for Italy?

See www.trenitalia.com/en/index.html. They allow 60 days out for booking over their website. Register FIRST...then, after receiving your password via e-mail, go in and book. If available, you would be eligible for Amica fares, which are 20% off regular price. OF course, you can enter dummy dates to get an idea as to prices.

Other than the Amica fares, there is little advantage to making train reservations in advance.

Agree with Frank. Wait until you're in Italy, then buy them at least a day in advance to get the Amica fare (20% discount on longer routes). If you use the kiosks in train stations to purchase tickets it will ask you for seating preference (window or aisle) on trains that require reservations, and will make the reservation when it issue the ticket.