Ciao Erik,
I think the most important thing to also consider is the layout of each of the towns and know that the ZTL is there to discourage unnecessary traffic through HISTORICAL CENTERS where all of the tourists are out and walking in (and are still inhabited by residents, of course).
Lucca's historical center is within the walls, the rest is more recent and not part of the ZTL - just don't try to drive into the center and you'll be fine! The walls are enormous, there is no way to do a wrong turn in as there is a street that goes all the way around and you are not forced to turn in. Parking is easy to find outside of it.
Siena is also a walled town. Most of the entrances into the city are through large doors along this wall. The ones I have personal experience with are the ones to the west and south of the town - there was always a large square right in front of the wall entrances where you could turn around and there was street parking leading up to this entrance or away from them. Once we went under the doors, there were signs to get to the paid parking lot to the right (if you went straight there was a stoplight and signs marked ZTL so it was pretty easy to figure out which way to go). We went into the parking lot and right out again, not paying anything to do the turn to go back out of the entrance! The paid parking lot was NOT within the ZTL. I've seen two other lots, they were the same way - there was a way to reach them without entering the ZTL.
If you've driven in Europe already, then you already know there are a lot of ONE way streets. When in doubt, follow signs to parking lots as they usually are NOT within the ZTL since they are for visitors who don't have access!
Pisa and
Rome are a little harder to determine boundaries but in Rome, you generally won't want to drive anywhere - too chaotic and parking is a nightmare. So park and then take bus or metro or walk.
Pisa and Florence - as long as you keep to the main boulevards AROUND the center, you are not in risk of entering the center.
Florence used to be a walled city, the main boulevard that surrounds the center is actually where the old walls used to stand. The map seen on the page Elena linked above highlights what is the ZTL - there are some entrances where you CAN turn onto another street and thus head back, there are others which don't offer the option. You can pretty much drive into Florence and just go around the center and park - all of the paid parking lots (except for the SMN and San Lorenzo market ones) are OUTSIDE of the ZTL so you can easily drive to them without ever risking entering the ZTL. That way you are parked within walking distance of all of the sights. Take a look at the parking lots on this page:
http://www.visitflorence.com/florence-maps/parking-lots-in-florence.html