When packing for a trip to Rome, keep in mind the saying: "When in Rome, do as the Romans do". Most women in Rome wear dresses all the time and the men wear slacks. People in Rome usually dress up all the time. If you want to fit in a little, just pack some of your nicer, comfortable clothes.
Bring shoes that are already broken in for your long walks around the town. It is advisable that you bring two pairs of shoes to Rome. You'll do more walking in Rome than any other vacation city in the world. One shoe may start causing problems after a day or two of walking, so it is nice to have another pair to switch to.
For toiletries, email your hotel and ask what they have available for you and bring whatever they don't supply yourself.
A camera is always a must bring for any trip. Bring any camera that you prefer.
Make sure that your luggage has some room for your souvenirs. However, if you plan to buy big, bulky stuff, it is advisable that you pack a collapsible bag inside your luggage.