“You can often purchase advance or special-entry passes through a
website affiliated with the museum or its city. Dodge the queue at more
than fifty English sites, including the Tower of London and Windsor
Castle, with London Pass (visitengland.com).
Likewise, Paris
Museum Pass(parismuseumpass.com) -->
allows you to jump to the front of the line at the Louvre and sixty
other monuments in the City of Light. For Berlin’s recently opened Neues
Museum, book online at neues-museum.de for timed admission. Avoid the long wait at the Sistine Chapel and Vatican
Museum by prebooking your visit through museumrome.com (two-hour private tours from $70 per person). And the Museo
del Prado (museodelprado.es; -->$10), in Madrid, offers discounted advance admission on its website.” —T+L
arts editor Mario Mercado.

Rarely in the US have I experienced long lines to enter museums (major exhibits often issue specific reserved tickets)---not so in Europe! I recommend you follow the tips on this article from Travel + Leisure!