From the Coliseum to the Domus Aurea, together with the Borghese Gallery
There are a couple of must to be seen sights of interest in Rome that should be booked in advance: some of them because the reservation is mandatory, some of them because they are extremely busy (some sort of bottle necks of the tourism in Rome) and so there are some skip the lines strategies that can be followed just buying the tickets ahead of time. Some sights need more than a month of notice in peak season, to be sure to find full availability on line.
The first to consider is definitely the Borghese Gallery: here the reservation is mandatory (and you reserve and you purchase the ticket). You can do it on.line (www.tosc.it) or by phone calling the dedicated number which is (+39) 06 32810. The cost of the ticket if you buy it calling is EUR 22,00 per person. On-line is slightly more expensive (you pay a pre-sales fee)
The second with mandatory reservation is the Domus Romane di Palazzo Valentini, an interesting archaeologic site on the Foro di Traiano Area, with very investing explanations about the everyday life during the Ancient Rome period. A site that should be visited. For the booking and purchase of the entrance (that goest together with a guided visit) you can use the same number as the one for the Borghese gallery: (+39) 06 32810.
The third with mandatory reservation is another archeologic site, the Domus Aurea (the Villa of the Hemperor Nero) . It is and interesting working in progress archeo site, with limited access. So the reservation is mandatory. A must see for Roman History lovers. Use the coo-culture website
The first with a non mandatory in advance reservation, but definitely the busiest (so mandatory the reservation if you do not want to stay hours queueing) is the Vatican Museums. Here the problem is not only given by the amount of people queueing, but also by the number of people who visit the museums and that you will be sharing the space with inside the museums itself. Here the strategy (which is a bit pricy but worth) is to go on-line and buy an early in the morning entrance (an entrance before the ticket office opens, at 9:00 am). Vatican Museums Official Website
The second with a non mandatory reservation but definitely busy is the Coliseum. Here you can spend hours queueing and since the entrance to the Coliseum is limited, the reservation on-line is almost mandatory. If you wish you can also buy the entrance with the visit of the last ring and the undergrounds. For this last option the reservation is mandatory, since the entrance to those areas is allowed only in small groups with a guide. www.coopculture.it