My very accurate predictor of onset of turbulence is about a minute after I get a meal or drink on the tray table in front of me. It's not been bad enough to result in me wearing any of it yet, but I've been on a few flights where there's been an announcement for cabin crew to take their seats immediately. (Cue to hold on to the drink and possibly put the lid back on the food tray.)
I would echo the comment from 747-8driver above about the seatbelt sign. While there are clueless people who get up at obviously inappropriate times, if it's been nice and calm for a significant period in cruise and the sign is still on, people start deciding that it's been forgotten, whether that's the case or not. Remember that in the back you can't see what's in front and the fancy instruments aren't visible either. Having said that, I still dislike being asked to get out of an aisle seat to let someone out of one of the other seats when the sign is on.