Check out www.onebag.com . This website has a good packing list, as does Rick Steves’ website, www.RickSteves.com . For more than a month, you may not be able to get everything in one bag, but it is a helpful place to start! And you will not lose your bag.
Things that are either difficult or impossible to find here in Italy, that you might miss from home:
Medicines and perscriptions (Ritalin is not sold in Italy, for example)
Zip Lock bags (yes really!)
Sun screen in Winter is harder to find
Bring copies of your prescriptions along with you–if you need a refill you can take them to a farmacia and they will fill your prescriptions either immediately or within a couple of days–often more inexpensively than stateside prices.
Also bring any medical records along with you. Italians are expected to carry their medical records from doctor to doctor. If you have a copy of your last dental x-ray, bring it along just in case– or ask your dentist to make you a copy (usually for aroung $5). It is a good insurance to have.
And speaking of, especially if you are going for a longer stay (over three weeks) consider buying travel medical insurance. There are several to choose from. I use American Express Medical Travel Insurance (an Amex card is not required to buy the insurance). Some features are… it provides in-country liaison personnel to help you as well as medical evacuation plans, and pays in-country expenses for you while you wait for your own insurance’s reimbursement.