I called 2 weeks ago and reserved our tickets to both the Accademia and Uffizi for a visit in Mid-July. As a previous poster stated, there is a 4 Euro/tkt. booking fee for each of these museums. The Accademia also has a special exhibit at this time so these tickets, which I believe are normally 6.50 Euros, go up to 10 Euros, plus the booking fee of 4 Euros for a total of 14 Euros/tkt. You have no choice re. the special exhibit fee. The Uffizi tkts. were 10 euros plus the 4 euro bkg. fee for a total of 14 euros/tkt.
You can go to the official museum website and check the number of slots available for the day you would like to visit. Then if you call shortly after you check this you can give the operator the exact time you would like. My entire call lasted about 10 minutes including the time I was on hold. I have int'l calling on my cell phone and the per min. charge isn't that bad.
In Rick's Florence guidebook update he mentions trying to book through the third party website, www.weekendafirenze.com. This website, does charge an additional 5 Euros/tkt IN ADDITION to the ticket price and the 4 Euro booking fee. I guess I would try to avoid booking through this website if at all possible.
If people are still having trouble booking though the official museum website, I would recommend just calling.