I started by reading reviews on the companys, siting specific drivers and then reading some reviews on them, after that it really came down to cost. I haven't come across bad reviews, but after reading the reviews I had a sense of which driver's personality would fit with my family.
For example: Fabrizio with allarounditaly is very funny and engaging. Michele with revelation tours in villefranche, is very talkative - he will talk 8 hrs straight, but very informative and very good with what he does. Some people would rather not have someone who talks the whole time, we didn't mind, in fact it was great. Nikos w/Santoriniday tours brings his camera and takes pictures of your family all day and hands you a CD at the end. Marcello with seesorento is very good but he's very antishopping and hates the tourist traps. He wants to show you the Italy of an insider. You will just need to be clear with what you want to do and he will accomodate, if it means shopping, he'll take you shopping.
In Florence we didn't use a guide but chose a driver....www.tuscanybytaxi.com since I had a good feel of what I wanted to see and do.