Try to pack for your whole trip in one bag, especially if you're going from one place to another. It's such a pain to lug around three different bags.
Venice is NOT the place for roller bags because a) the alleys are so narrow and you will be dodging people b) you have to constantly carry them up and over all the bridges and c) everything is bumpy cobblestone. Venice is actually not a great place for luggage at all - there's a trick though: you can hire a porter to meet you at the train station or at your hotel and he'll load all of your bags onto a cart and wheel them to your next destination for you. Ask the front desk of your hotel how this works - if you have a lot of baggage, it can be worth it.
Watch out for pickpocketers in crowded, touristy places. When we announced that we were stopping in Naples for lunch during our big Italian trip, my mother was so.worried. She thought we were going to get pickpocketed, robbed, hit by a taxi, etc. While Naples is known for pickpocketing, you just have to be aware, not scared. Men should keep their wallets in their front pockets instead of their back and women should carry bags that zip closed so that no hands can wander in. Keep track of your belongings and be aware of your surroundings and you should be completely fine.
You will find hotels in Italy that don't have air conditioning. It can get quite hot in the summer, especially in cities, so if this is something that matters to you, check and make sure that your hotel has AC.
Many hotels also don't have elevators. If you're traveling with elderly or handicapped travelers (or lots of baggage), make sure there is an easy way to get up to the fifth floor.
Your hotel should be able to give you a good map of the city, better than what you'll find in a guidebook. Have someone at the front desk circle where you're going - restaurants, bars, sightseeing spots - they'll be much easier to find.
Research local foods ahead of time (or ask your waiter) and order at least one local dish. Each region in Italy has different specialties - different pasta dishes, types of meats, seafood, different sauces...
If you don't feel like spending a lot on a fancy bottle of wine, order table wine (vino della casa) because it's always delicious and inexpensive - and everybody does it.
If you're trying to save money on food, look for local markets. There are usually great markets in any city you'll visit (Mercato Centrale, in Florence for example) - getting lunch here saves money and you can take your food and eat outside in the shade. Also small sandwich shops - in Rome, we ordered sandwiches from a small cafe and took them to the Spanish Steps for a picnic.
Aperitivo - the Italian equivalent of American "happy hour" - can be a big money saver too. When you order a drink, it will come out with food - sometimes it's something simple, like little bowls filled with green olives or potato chips, but at many places you can also snack on pizza bites, meats, cheeses and breads - all for free!
Italy can get so hot in the summer - take a water bottle with you and fill it up as you go. In every city, you'll find fountains with drinking water and having a water bottle is much less expensive than buying bottles of water wherever you go.
Wear sneakers or really good walking shoes. Everyone in Italy, real Italians, do this for a reason. There are lots of cobblestone streets and you'll be doing a lot of walking and you'll want to be wearing good shoes.
Book tickets ahead of time. Many sights can get really crowded in the summer and it's best to already have tickets when you arrive so you don't have to wait in a long line, or get shut out entirely. Your hotel can help you book (or a travel agent, if you're using one) or you can book them yourself online.
Carve out nap times. It's the worst to be sightseeing when you're tired and you won't appreciate what you're seeing anyway. Make time to just relax...it's supposed to be a vacation, after all.
There are some sights that you can see at night (sometimes they're even more spectacular in the dark, when they're lit up) when it's cooler: the Rialto Bridge in Venice for example, or the Trevi Fountain and Spanish Steps in Rome.
And - if you're going to Venice - here are our Venice favorites...