Get a helmet or eye glass mounted mirror because it allows you to see a wide area behind you by moving your head. Bar mounted mirrors cant see the traffic if there is a curve in the road. You dont have to worry about cars passing on the right if there are no cars turning left. If you are about to pass a car making a left turn, use your mirror.
The flag, reflective gear and flashing lights all help, but are no substitute for keeping alert to whats going on around you. Ride in a manner that wont give any motorist a surprise - 99.9% of the motorists are not out to get you, and there isnt much you can do about the others. Good luck