You might think this is obvious. If you do, you have not been driving in front of me lately so I thought I'd give some tips and hints.
  1. BEFORE you turn.
    The signal is meant to warn the driver behind you that you are ABOUT to turn, not that you are turning at that exact moment. That gives the driver behind you time to act/react accordingly and peace upon the Earth prevails.
  2. BEFORE you turn.
    Not AFTER you've been sitting in the intersection refusing to move, forcing the driver behind you to honk, thus revealing your secret plan to turn.
  3. Even if you're in a turn-only lane.
    Using your turn signal in the turn-only lane lets the drivers around you know that you KNOW you're in the turn lane and do, indeed, plan on turning. Without the signal, we can't be sure if you won't be one of those drivers that's going to try to force their way straight through the intersection, causing us to use profanity.
  4. When you want to change lanes while driving.
    Again, this is a prior-to situation. People are much more courteous about "letting you in" and it lets that pesky driver in your blind spot know your plans before you barrel into them.
  5. When you're in a lane of traffic that will split up ahead.
    Similar to the turn-only lane, using the turn signal in this situation lets the drivers around you know that you are paying attention and have a plan. Not using one doesn't mean that you're cooler than everyone else, similar to not wearing elbow pads while skateboarding, it just means you're an ass.
  6. The decline of the use of turn signals parallels the decline of manners and civility in our communities. The guy stubbornly sitting in the intersection without a turn signal on has never written a thank you note in his life. I say we take back this small piece of what used to be good about America and make it good again!