I always have a sunglass mounted rear view
mirror.
I can scan all the way across the road behind me with just a short turn of my head. It's way easier to merge into another lane if I can time my lane change to gaps in traffic. (But just before the move, turn your head and check anyway.)
And I'll move over to the left tire track in my lane if there's no cars within a few hundred yards. Then I have a little more room when someone darts out from between parked cars or a car pulls out without looking.
Street Riding
I liked this
Bicycling Street Smarts web site.
From the chapter
Where to Ride on the Road:
"By riding a safe distance from roadside hazards, you increase your safety. When you ride correctly, the motorist in the driveway (a) sees you; the motorist overtaking you (b) will not take the easy way out and skim by your elbow; and the car door (c) is no threat."
And a couple of videos that emphasize taking the lane when needed.
The Rights and Duties of Cyclists
How to Ride Your Bicycle in Traffic. This rider avoids filtering up on the right, since cars often make sudden right turns without looking. (Turn on youtube annotations.)