Supervising a beginner in traffic
I ride behind: she sets the pace, concentrates on the road ahead. Being behind I can watch, control the distance between us, speak advice, and manage (i.e. observe and give hand signals to) any vehicles approaching us from behind.



Edit: I just found a quote on the front page of The VEHICULAR CYCLIST (December 2011)


  "I’ve taken three cycling courses since becoming a community officer with the Ottawa Police Service and am on schedule to become a cycling instructor this fall. At the end of every course, I’m absolutely amazed at all the bad habits that I had taught myself throughout the years, ...".


http://www.toronto.ca/cycling/safety/index.htm mentions "CAN-BIKE" courses. One idea might be to take that kind of lesson: I'd at least see how they teach it.