Need to buy train tickets in advance?

I bought most of my tickets the day of travel at the train station. Sometimes I went to the window, and sometimes I used the self service kiosks. A few times, I bought tickets ahead of time at a tourist info center or at the kiosks. There is an English option on the kiosks, and it walks you through the process.
If you are good at keeping track of where things are and know exactly what day you are going where, and want to make sure you get a seat on the fuller trains (hard to know in advance which ones that will be) you can buy all your tickets at the same time. The ticket agents at the train stations often speak English. Just tell them what days you want to go, and to where, and what time, and which ones you want to be round trip tickets. To make it easier on them, you might even write it down and just hand them your itinerary.
You can also do this through the self service kiosks in the train station.

Also..... if you buy multiple tickets, be sure to have the right ticket for your destination in your hand and PUNCH IT in the little machines that are all over the train stations (most of them are outside on the platform). They are little timeclock looking things, and do exactly that: you put in one end of the ticket and it time-stamps it. YOU MUST DO THIS or you will get a BIG fine! We forgot just once and that's all it took! A 50 euro fine later, each, and we remembered to validate our tickets after that!!
Be sure you do NOT punch any ticket you are not using that day. ONLY punch tickets you are using right at that moment. There is a time expiration on tehm once you punch it if it's a regional train (slower, cheaper, marked with an "r" on the schedule). If you get a reserved seat on a Eurostar train (an "ES" train on the schedule), your ticket is only valid for that day at that time so be sure to use the right ticket.
We often chanced it and got burned once; we wanted a Eurostar train from Verona to Florence and it was full. We either had to take a regional train or wait for another ES train; so sometimes it's best to buy them at least a few hours ahead of time.

Don't stress too much about it; it's really not as complicated as it may seem, and not a huge deal if you don't get THE train you want. There're lots of trains, lots of options, and people who speak English at the stations to help you out if you need it.
have a great time!

As suggested, unless you are coming during a holiday week (such as Holy Week or XMAS) you'll have no problem getting tickets on the trains from Florence to Rome (and return). I'm not sure why you would want to make three trips (if I read your note correctly) back and forth to Florence... that can get quite expensive on the high speed trains. I use the Trenitalia site to see the best routes, times, and trains... I would recommend you print it out and take it with you so you know your best options.

The high speed trains take a little more than 1.5 hours, but some of the regionals can take up to 4 hours! So you'll probably be on the Eurostars or the AV trains (the fastest!). These "fast" trains all have reserved seating so no need to validate on these trains as you will have a seat number. There are many types of tickets available. For more info on the types of tickets, CLICK HERE

If you can get your credit card to work - quite difficlut for a US card holder - then you can get 5% on specific fares booking on Trenitalia. You can also get breaks for advance purchases.

If you need more help on purchasing through the kiosks and a step-by-step example,
CLICK HERE.

If you need more on just riding trains in Italy (RS has great support for this), then this also may help,
CLICK HERE

Good Luck!
Ron in Rome

Thank you all for your tips! Sounds like we don't really need to worry about getting tickets before we leave since I'll be in Italy mid-October.

Ron -- I was just looking at info on your website earlier! Don't worry, we're not going back and forth to Florence =) We're flying into Rome for a few days, then taking the train to Florence. Will be based in Florence for 5 days before flying home from Florence. Therefore, we really only need two train tickets - one from Rome to Florence; the other from Florence to Assisi (for a day trip).

Thanks everyone for the advice! It really does help to know we can put this off until we get there. Definitely allows more flexibility. Thanks again!

Let me clarify the rules about "punching" tickets for Trenitalia trains: you are required to punch tickets only for the unreserved "regionale" ("R") commuter trains, and not for any other train category (AV, ES, IC, ICN, etc.) which have automatic required seat reservations (since the ticket for these reserved trains is valid only for the specific train selected). Nevertheless, you may punch ANY train ticket in the little yellow machine, without adverse consequences, even for those trains for which "punching" is not required. If you enjoy the experience of "punching" the ticket, go ahead, it won't hurt!

I just got back from Italy and never had trouble with getting train tickets. I purchased the tickets half hour before departure. I took the Eurostar and the regional trains and there were plenty of seats even with people taking extra seats for their luggage.

If you buy tickets the day before and travel by IC train you can get the Amica discount which will save a few dollars.The machines are very easy to use.Roninrome describes it very well on his website.