The most obvious suggestion for avoiding crowds in Rome is to go off-season. Well, I am here to tell you there is really no off-season in Rome. Even in the dead of winter, you will see large tour groups and lines forming at all of Italy's most famous attractions.
Other travel tips are to go super early and be the first ones in or enter the site an hour before the last admission (in most cases 30-60 minutes before closing). Of course, this time actually depends on how long you want to stay in the museum or site before closing.
Here are a few more tips for some of Rome's most popular sites that might help navigate the crowds or see them crowd-free.
Sistine Chapel
if you have the money and appreciate once-in-a-lifetime moments book the Keys to the Sistine Museum. You actually meet the gatekeeper of the Vatican who opens the doors every morning. He gives you the key to the doors of the Sistine and you enter the room for viewing with just you and your group. Breakfast is served after.
Another tip is to book the earliest entrance time into the Vatican Museum and go straight to the Sistine Chapel and then backtrack to the rest of the museum rooms you passed along the way. Most people and tour groups view all the rooms on the way to the Sistine Chapel which is right before the exit. By going to the Chapel first you beat everyone else that booked the early Museum entrance time.
St. Peter's Basilica
Currently, it is free to enter the Basilica. Keep in mind it is closed on Wednesday during the Pope's visit to St. Peter's Square and during Mass times. I entered the Basilica right when it opened and right before it closed. Both times we were practically the only ones inside. Be sure to allow time to go through security before entering. Again the lines for security first thing in the morning and right before it closes are considerably smaller than during the rest of the day.
Colosseum
For a higher view above the crowds - if you are coming via the metro exit and take a left and up the stairs. Where Via Niccola Salvi and Via degli Annibaldi meet. Even fewer crowds are farther to your left down Via Annibaldi to "Giardinetto di Opia" -a tiny garden nobody other than Romans know about. If you are a walker/hiker you might enjoy the view from Palatine Hill. It's a big park with lots of locations to view the full Colosseum as well as other Italy landmarks like St Peter's Basilica and the Grand Synagogue of Rome.
For visiting inside the Colosseum, I suggest an after-dark tour. After-dark tours have to be scheduled with a guide. See the outside at sunset for stunning photos and then meet your guide to see the interior with as few as 15-20 people at a time.
Trevi Fountain
If I know I am having a late dinner in Rome and I am in the area of the Trevi Fountain, I make an effort to stop by. In Italy, it is not uncommon to finish dinner after 10 PM or even 11 PM. Stopping by at these times usually allows you to get some photos in without strangers in the background. Plus evening photos with the warm glow of the lighting make for very romantic photos. If that's your thing (wink).
Conversely, if you are an early riser the Trevi Fountain at sunrise offers a pink sky for a background. It is simply stunning.
This last tip is for when you can't avoid a daytime visit or just want a different view of the fountain. If you are standing dead center facing the fountain, do a 180 and you will see the United Colors of Benetton store. Enter the store and make your way to the 1st floor. There you will see a window overlooking the Piazza di Trevi and the fountain. The store is generally open from 10 AM - 8 PM.
Borghese Gallery
Schedule your visit to the Borghese Gallery ahead of time. Chose a time for Tuesday -Thursday. Weekends are the most crowded. The museum itself says 11 AM is the most popular time of day for visits, so arrive before then or later in the afternoon.
Roman Forum
Instead of mixing with the crowds inside Italy's most famous Roman Forum head up the grand staircase to Piazza del Campidoglio. Once in the Piazza and facing the big Fountain of the Goddess Roma walk to your right. The street which looks more like a walkway is called Via del Campidoglio. Continue under the arch and to a viewing point for spectacular panoramic photos of the Forum and the Colosseum behind it. You can also do a Google Map search for the terrace- aptly named "Terrazza sul Foro."
