You can now make your reservations online through the official museum web site. It appears the charge is the same as calling.

http://www.polomuseale.firenze.it/english/

We have been in Rome and Florence in late November-early December. We encountered very few lines at any of the museums (for example, we walked right into the Uffizi).

Thanks for all the great advice.
I was able to get reservations for the Uffizi and Accademia for the the times that fit into our schedule. I was getting a little worried when my hotel said everyone was on Holiday that week!