There are some new traffic lights up that let you know when it is okay to drive into the ZTL. The pictures below are from the entrance in front of the Biblioteca Nazionale (I am not how many ZTL entrances have these lights installed). The current mayor of Florence, Matteo Renzi, has announced new plans to make the whole ZTL situation simpler and more friendly to tourism. We'll see how that goes - for now the signs are a decent start.
Here is the location of the above sign:
View Biblioteca Nazionale ZTL Entrance in a larger map