In the Summer time 

If you’re visiting Rome in the summertime, make sure you pack the right clothes. Typical temperatures hover around 35 degrees celsius (about 95 farenheit) and the humidity is well above 80 per cent. It doesn't cool off much at night, either; from early July until mid-September you can expect nighttime lows around 25 celsius, or 77 farenheit. Even Romans complain about the heat ("caldo") and humidity ("afa"). In fact, the city empties out in August when residents flee to the beaches or mountains.

So what do Italians wear when it’s hot? Men will tend to sport lightweight shirts and trousers. If you want to try to pass as Italian, trade khakis for more colorful chinos and wear them with a regular blue dress shirt, sleeves rolled up -- and add trendy sunglasses! Women favor lightweight dresses and skirts. If you aren't used to the heat and humidity you will likely find jeans very uncomfortable -- even denim shorts and skirts are no fun. Woven fabrics and looser clothes will be cooler than knits or anything close-fitting; it may sound counterintuitive but a lightweight cotton tunic is much more comfortable than a knit tank top. Big heavy belts are to be avoided for the same reason. A broad-brimmed hat can be a good idea. 

In the Winter Time 

During the winter, temperatures can get close to freezing, so sweaters and coats are necessary.  A heavy sweater and a water resistant jacket with hood should be fine for most circumstances. A couple of warm, fashionable scarves will take up little room in your suitcase and add personality and a lot of comfort when it's cold and windy. It rarely snows in Rome, but the rain can be torrential, so footwear with at least some water-resistance is advisable. Skip canvas sneakers in winter, and remember to bring an umbrella! Roman women use cold weather as an excuse to sport fashionable headgear; you'll see everything from floppy knit berets to mink Cossack-style hats, so if hats are your thing, bring them. Men wear various styles of caps too.

Shoes and Walking Needs 

Remember that Rome has some tough streets to walk on. Cobblestones are common in the historic city center. Even the paved streets are uneven and have potholes to dodge -- and everything is much more slippery in the rain. Roman women somehow manage in spike-heeled boots, but they're experts: to make sure you can get around easily, wear comfortable, low-heeled shoes.

In summer, many tourists wear sport sandals -- not the most beautiful of shoes, but very practical for doing lots of walking when it's hot. Tennis shoes, sneakers, and flat loafers or ballet slippers are also good choices, as long as they have decent arch support and a little padding; a shoe with thin soles can become unbelievably painful if you're walking on cobblestones. Open toed sandals may become troublesome if you are walking in a gritty or dusty area, like Ostia Antica. It's best not to buy new shoes for the trip, because they may not be as comfortable after two hours on cobblestones as they were in the shoe store. Wear trusted, comfortable, well broken-in shoes. If you're prone to blisters, you can spray your feet with antiperspirant to reduce friction from sweating in summer.

Special Considerations for Churches and Places of Cult 

When visiting religious sites remember to cover your shoulders and knees. Long shorts (below the knee) and capris are fine; short shorts and miniskirts are not permitted. Women often carry a scarf to put around their shoulders when entering a church. However, covering your head is not required. Sandals are also fine; many monks and nuns wear them. Men have to remove hats when entering a church. 

Vatican Dress Code

The Vatican has a dress code which is enforced. People with exposed shoulders, knees, midriffs, or (for the ladies) cleavage, are denied entrance. Same sort of respect is expected in all places of cult. Here is a picture showing the vatican dress code: http://sphotos.ak.fbcdn.net/hphotos-a...

Dining Out 

Italians tend to dress more formally when dining out than Americans, but you don't need a dress or a jacket and tie unless you plan to go to very elegant restaurants. A polo shirt with trousers or a skirt and top are fine in summer, and sweaters and trousers are fine in winter. At casual restaurants, which are very prevalent in the historic center, t-shirts and shorts are acceptable. Usually, there's no need to wear different clothing at meal times than what you wear when sightseeing.