Trains

Trains are frequent. Check trenitalia.com for schedules, then head for Termini station at least a day before and get tickets from one of the machines. If there are any left, you may get an Amica fare, which is 20 per cent off. Baggage storage is on your own hook, above the seats or at the end of the carriage.

Take the train. For timetables and fares, go to www.trenitalia.com/en/index.html. Enter Roma and Firenze as your departure and arrival points. In Italy fast trains generally require reservations. The reservation is included in the price of the ticket. Reservations aren't even possible on regional trains.

Buy your tickets when you arrive in Rome. If any of the alloted seats are still available, you can get Amica fares (20% discount) up to 24 hours before departure.

You are responsible for your own luggage. Larger luggage goes in the bins at the end of the car. Keep an eye on it when the train stops at a station. Use the overhead racks for smaller luggage.

I would modify that suggestion on the train reservation time. Make it as soon as you are sure of your dates. Since you are doing just two cities, it is not likely you are going to suddenly go to say Sorrento instead of Florence so book now on Trenitalia's site to be sure and get Amica fare. 20% savings is worth it. If you are less sure, then wait. Otherwise, on the net, you can book it even before leaving home.

Oh, if you can't get Amica fare, then making early reservations is less important other than the fact that it insures you will be sitting together. We had problems getting seats together when we waited until the day of travel and that was October. You might have less trouble now.

You do not check bags. Since you are doing a "round trip", see if the Rome hotel will hold your extra bags and take only what you need for Florence. Bags can be placed at one end of car (special area good for big bags), overhead (you have to lift), or behind seats that are back to back (small bags).

We were staying about half a mile from the Rome train station, so we just went there a few days before our trip to Venice (via E* through Florence), and booked using the automated machine. It gave us the Amica fare option, although we had to take the train a little later than initially planned.
If you know what time you're going to want to take that trip, hit the machine after arriving in Rome before heading to your hotel, or at the ticket window. You can use the Trenitalia site to find out what times and options are available.