AlaskaCoho did a good job of explaining the mechanics involved. As others have pointed out, there's no good way to predict well in advance. That said, here are some areas where turbulence can almost be guaranteed, but it won't necessarily be bad:

North Atlantic, any time of year. Yes, winter is worst.

North Pacific, any time of year. Yes, winter is worst.

Central US, especially in spring and summer.

Flying along the Cascade/Sierra Nevada range (across, as well, but it will be shorter-lived).

Rocky Mountains, any time of year but especially in spring and summer.

Mainland US to Hawaii in winter. There have been reports of flights where the flight attendants kept their seats for the full 6 hours.

US west coast in the winter. Not so bad in other seasons.

Appalachian range. Among small plane pilots, it's widely acknowledged that the lower altitudes over the north-central part of the Appalachians (New York, Pennsylvania, Maryland, Virginia) are consistently the worst in the country for the presence (again, not necessarily severity) of turbulence.

Anywhere I happen to be flying! Don't get that many smooth flights anymore!