Avoid free entry: Many museums with entrance fees open their doors for free once a month. If you want peace and quiet, avoid it.
Attend Mass: As long as you dress appropriately, and attend until the dismissal, you will always be welcome to attend a church service and it can be one of the most evocative ways to see the great cathedrals.
Enter the Louvre at the Porte des Lions: This door at the extreme south-east corner of the museum is generally unburdened by queues. It also happens to be the quickest way to get to the 'Mona Lisa'.
Think laterally: For example, the great painted tombs in the Valley of the Kings in Luxor are often deserted during the hottest part of the day, when the tour buses whisk their charges off to lunch. Endure the heat outside, and you can visit the tombs in peace.
Go underground and topside at the Colosseum: For a supplement of only ¤1.50, you can escape the crowds and visit the newly reopened basement and top tier of the Colosseum -- otherwise closed to the public -- in a small guided group. Book several weeks in advance on 0039 063 996 7700, asking for a tour in English.
Book the Vasari corridor at the Uffizi: Lined with an exceptional collection of portraits, and you will enjoy a fascinating respite here from the crowded museum. Open at limited times; see polomuseale.firenze.it.