How to get from Siena to Florence

Take the train. Here's the website to look up dates and times (click on the GB flag in the upper right hand corner for English): http://www.trenitalia.com/

Are you looking at driving, bus or train. For train, www.raileurope.com, offers tickets on the Trenitalia line for $13 one-way.
By car it is about an hour north.

We took the bus from Florence to Siena, opposite direction. Cheap and easy. Multiple trips per day. You can inquire at front desk of your Inn or tourist info. stand.

This thread should help. It tells you exactly what to do: Bus from Siena to Florence

Here's the helpful part:
You can get the bus to Florence at Piazza Gramsci in Siena - a short walk from the historical center. .... You can buy tickets at the Piazza.
The buses run frequently - I recommend an express bus that only has a couple of stops between the two cities. Once in Florence, you are dropped off at the bus station which is just around the corner from the main train station and a short walk to the main tourist sites.

I never use Raileurope, it's a broker that marks up and selects schedules, you won't get all available choices from them and you will pay more.

I've only taken the bus. I understand that the downside of taking the train is that the Siena station is somewhat distant (a kilometer or more) from the historic center.

We also took the bus from Siena to Florence. It is convenient as the bus stops right at the historic center in Siena so you can walk to the bus stop and you can get off at Sta Maria Novella bus/train station in Florence. You can buy your bus ticket at tabac shops; there's one near the bus stop.

The bus is definitely the best way to go, unless your lodging is very close to the train station in Siena, which is far from the Historical Center of the City. I've taken the bus a number of times. Inexpensive and multiple departures/arrivals. You can buy your ticket when you're there at the bus station. No need to sweat this out beforehand. Just make sure if this is a "day trip" that you get that last bus back, and they can give you that info when you buy the ticket.