Last week in Paris, we spent a half day in Montemartre and would have skipped it altogether if we had been warned about the hoards of tourists we'd encounter. The crowds there were so thick at the top of the hill, it really wasn't worth being there.
As more and more special unique places get loved to death, travel guides can suggest less discovered alternatives.
For example, skip the crowds in Florence and stay in Bologna or Lucca instead. (You won't find Bologna, my favorite Italian city and the country's gastronomic capital, in Rick Steeves' guide to Italy. Maybe he is protecting Bologna from the rest of us.)
Avoid the hill towns congested by buses in the south of Tuscany by opting for the less discovered hill towns in the north of Tuscany (the Garfagnana). Or stay overnight in one of these idyllic towns to enjoy them after the buses drive off at 4pm.
Stay away from the congested sidewalks in Prague by going to Talinn, Riga and/or Vilnius in the Baltics.
You get the idea.
The Rick Steeves' Paris guidebook does a great job, for example, of suggesting many lesser known museums as less crowded alternatives to the best known few, in addition to highlighting the least crowded times of the year. But more alternative destinations would be a huge help, too.
What are your favorite alternatives for avoiding crowds that are worth recommending?