Here are The Traveling Professor's Top Ten picks for visiting the Vatican, Sistine Chapel, and St. Peter's Basilica

1. Check the Calendar:  There are no visits to the Vatican Museum on Sundays, during special events, or on religious holidays.
2. Go on Friday Night:  From April to October, Friday evening visits to the museum and Sistine Chapel are offered. Few people know about Friday evenings and even fewer people show up.  It is a unique opportunity to visit with few crowds.  Please note the Basilica is not open on Friday nights.
3. Book in Advance:  The Professor strongly recommends booking in advance to skip the lines.
4. Book a Guided Tour of the Museum, Sistine Chapel AND Basilica:  Guided tours are far superior to going on your own.  The major benefit of booking all three places on one reservation is that yo are able to go directly from the Sistine Chapel to the Basilica without exiting and having to walk all the way around to the Basilica and having to wait in line to enter.  Take my advice on this one!
5. Only Use the Official Website to Book:  Here is the URL:  https://biglietteriamusei.vatican.va/musei/tickets/do?action=booking
6. Beware of Pickpockets:  Unfortunately, they are all over the place.
7. Pay No Attention to "Skip the Line" Hawkers:  They can't be trusted.  Only book directly through the ticket office or official website.
8. Dress Appropriately:  I have seen people wait in line for hours only to be turned away because of their abbreviated wear.  No short sleeves or short dresses are permitted.
9. Consider a Visit to the Tomb of Saint Peter and the Necropolis under the Vatican Basilica:   This is an absolutely fascinating tour below the altar to visit the tomb of St. Peter.  Special arrangements must be made well in advance HERE, and they are separate from visiting the museum, Sistine Chapel and Basilica.
10. Have Lunch at Trattoria Perdincibacco:  It's a terrific place to rest those weary feet after a long tour.  Great pasta, cool drinks and a wonderful staff will help you relax and eat.