To answer my own questions:
light 3-bet/4-bet preflop? If someone is opening a lot of
hands, or folding to a lot of 3-bets, you can start bluffing preflop.
Bluff-raising the flop? Against someone with a wide preflop range and high continuation bet percentage, you can bluff-raise a lot of dry flops. Floating them to bet or raise the turn is another option. This is mostly about finding boards that miss their range.
Firing the second barrel on the turn? Opponents who call the flop with a wide range are typically good to second barrel on the turn. Scare cards are usually more effective than blanks, but...
Shove-bluffing the river? If someone calls the flop and calls the turn, they rarely have a super strong hand. There are exceptions, but most of the time they'll be hoping that you check the river. I like to disappoint them. If I fire the turn as a bluff, there's a good chance I'm going to bomb the river. Sometimes it doesn't work out (I shipped Q-hi on a J5525 board into AA the other day), but it works often enough to show a profit.
As with anything, bluffing too often can get you in trouble. But they never know you're bluffing unless you get caught.