Ideally, you should spend at least a week in Rome to even begin to experience it properly. But there is a lot to see in Italy, so it's understandable if you have to run. To be sure that you see everything you must and still have a chance to experience La Dolce Vita (which must never be crammed or rushed) here are some itinerary ideas. Be prepared to walk a lot!
-Start with a visit to St. Peter's and the Vatican. Make sure you make a reservation ahead of time (either by faxing the Museum or arranging a tour guide through a company) to avoid wasting your time in line for the Vatican Museums.
-After your visit, take a leisurely stroll past the Castel San Angelo (a.k.a. Mausoleum of Hadrian) and cross the Tiber via the Ponte San Angelo.
-Continue to the area around Piazza Navona. Stop and have lunch at one of the many (not cheap!) restaurants right on the Piazza, or try the restaurant Cul de Sac, just around the corner at Piazza Pasquino #73. You'll find more wines than you can imagine and delicious small plates.
-After lunch head east toward the Piazza della Rotonda (stop at Santa Maria Novella Profumeria on the way) to visit the Pantheon.
-After you have feasted your eyes upon the wonder that is the Pantheon, head north to Giolitti's for a much-deserved gelato.
-Continue east to Piazza di Spagna. Get distracted by the designer shops along the Via Condotti on your way to climb the Spanish Steps.
-By now your feet will be tired. Head back to your hotel for a nap before dinner or up to the Pincio for some cool breezes and refreshing views.
-For dinner try restaurants near or on Campo dei Fiori. Soak up the evening light. Have another gelato.
-Head to the Roman Forum when it opens to avoid the crowds. The official opening time is 8:30, but if you show up a little earlier, you'll probably get in.
-Purchase your ticket to the Colosseum at the entrance to the Palatine Hill. You'll save tons of time waiting in line.
-Head back through the Forum (or go around the Palatine through the Circus Maximus, which is now a big open field) over to the Capitoline Hill (Campidoglio). Have a coffee to restore yourself.
-Visit the Capitoline Museum. Don't miss the equestrian statue of Marcus Aurelius and the amazing view of the Forum.
-Head over to the Jewish Ghetto for lunch. Pass by the Theater of Marcellus on the way.
-If your feet can handle it, walk through the Campo dei Fiori, up to the Piazza della Rotonda, and over to the Piazza Borghese to browse the antique books and prints stalls.
-Have a gelato. you deserve it.
-Before dinner (preferably after you have had a chance to rest your feet for an hour or two) visit the Trevi Fountain. It's best seen at night, so don't go out for dinner too early!
-Hop in a cab or take a bus over to Trastevere for dinner. Relax in Piazza Santa Maria and get lost in the winding streets.
-If you are up for it, walk back to your hotel through the historic center. Rome at night is magical.