1. Pack only one carry-on sized backpack.
Any travel guru worth their salt espouses the virtues of traveling with one small backpack. Less is more, especially for Italy. Most people visit Italy during the summer which makes packing light especially vital. Italy is HOT from June to September, and everything you bring will have to be lugged around airports, train train stations, stairs, and hilly streets.
I've traveled overseas for months at a time with only one backpack and never felt under-packed. Personally, I would pack three to four shirts, one pair of shorts, one pair of light pants, a sturdy pair of walking shoes, and flip-flops for a summer trip to Italy. Leave any item you might need at home; it's easy to purchase necessities as you travel.
2. Pack a Rick Steves Italy Guidebook. No, I'm not being paid by Rick. His guidebooks are simply the best. I have used other guidebooks when traveling Europe (such as Let's Go, Lonely Planet, and Eyewitness) but they were not as helpful. In my experience, Rick is spot-on in using simple language to describe hotels, restaurants, and attractions. The best dining and accommodation experiences we had in Italy all came from Rick Steves suggestions. We also found his Italian phrasebook helpful, but isn't always necessary considering most Italians in tourist areas speak English. If you are interested in buying one, Amazon usually has the best price.
3. Pack a small Point-and-Shoot Camera
Unless you are a professional photographer, there is no need bring a SLR camera with all the fixings (lenses, tripod, case) on your trip. For one, you will look like a ridiculous tourist with the thing hanging around your neck. Second, most name brand point-and-shoot digital cameras are small, easy to carry, and take quality pictures. My wife insisted on bringing her SLR camera to Italy this past summer, but ended up leaving it behind during our day trips because it was a pain to lug around. In fact, our best pictures (including every photo on this blog) were all taken with my $100 camera. 
