I have noticed that many people do not set thier rearview mirrors to the best possible angles. My dad taught me to set them so I "just see down the back end of the car". But this really limits the field of view. I was always looking over my shoulder to see cars in my "blind spot" just over my shoulder. A couple times I almost didn't see one passing me before I pulled over.
I found a better way.
All cars have a center mirror, which has a fixed field of view. The side mirrors can be moved, and should show the field of view just outside of the center mirror's view. Move the angle of the side mirrors out so they just barely overlap. If you do this on the highway, let some cars slowly pass you. You can watch them in the center rear mirror, and as the move out of the rear view mirror, they should show up in the side mirror. Then, as they come out of the side mirror field of view, you should be able to see in your own peripheral vision. This will give you a better coverage of the area around you. With your mirrors like this you will be safer by being able to see cars all around you without having to crane your neck around!
Too much overlap and narrow view
Better mirror angle and increased view
See also this site: http://www.smartmotorist.com/mir/mir.htm