The best advice I can give you is what some others have said here, check out the turbulence forecast map (they have a web site and an app for iOS). I've used it for years and find it to be very accurate 80%-90% of the time, and even more accurate than what the pilot tells us over the intercom at times.

Other words of advice, at least from my experience:
You'll feel rough air more in smaller planes - so try to book on A320/321's more than say, anything in the ERJ fleet if you can help it (short connections obviously make this unavoidable).

Flying early in the AM or late at night tends to be a bit calmer, which of course is all relative to what the weather is doing.

Long haul flights can usually fly around forecasted turbulence more effectively, whereas shorter flights don't have a lot of point A to B wiggle room.

Finally, keep this is mind: most flyers will only experience light (routine) to moderate (memorable but not PTSD inducing) turbulence in their lifetime. Severe turbulence is quite rare, regardless of what the map says. Extreme turbulence is rarer still.

If you have any questions on how to read the turbulence forecast map or any other advice, contact me and I'm more than happy to walk you through it.