We've been to Italy twice now in summer and it is our favorite country. While it would be nice to be there off-season, it is by no means horrible to be there in August. Some tips:
1) Yes, decide now on your itinerary and book hotels for each night. What you lose in flexibility you will gain by not spending your precious time looking for a place to stay. Stay 2-3 nights in each place and DON'T try to see the entire country at once. Consider carefully going to smaller places that could be overwhelmed by summer traffic - we've not been to CT yet, will probably save that for a different time.
2) Get out EARLY on the days that you will see big sites - we loved our walk through relatively quiet streets to St. Peter's (in late July). When we came back out after our visit I couldn't believe the mobs of people. Glad we had seen it before it was so crowded.
3) Research museums where you can get tickets/reservations ahead of time (Uffizi) and book one or two.
4) Head back to your cool, dark room after lunch for rest and to escape the heat of the day. You need to get back out in the late afternoon and dinner isn't until 8:00 pm anyway so you will still have a full day!
5) Carry a water bottle and keep it full (yes, you can drink the water, including many public fountains). Take a short break after your first major site/2+ hours for a cool drink. Eat a real sit-down lunch every other day or so - great food, wine, cafe - you get the picture.
6) Pompeii is hot and dusty. Plan accordingly.
7) Many small hotels in Tuscany have pools - take advantage.
8) Leave your jeans at home. Not doing the "can I wear jeans in Europe" debate - but really, summer in Italy is HOT. Jeans are heavy & HOT. Lightweight clothing is the way to go. Plus you can bring more (shirts, not bottoms) and you'll look and feel great.
9) Eat gelato every day. Repeat.
10) Enjoy a somewhat slower pace. You really will be miserable if you are trying to do too much in the heat and crowds.