Luckily, getting your mirrors just right requires no special tools or skills, and you'll only look a teeny bit silly for a few seconds at most. Don't worry. After all, there are pictures of you immortalized on Facebook doing far sillier things than this -- and this silliness is in the name of safety. Here's how you do it:
Lean to the Left: To adjust the driver's side mirror, lean until your forehead touches the window glass. Then adjust the mirror until you can just barely see the slightest sliver of the side of your car. When you sit up straight in the driver's seat, you shouldn't be able to see your own car at all. Why would you want to? When has the rear fender of your own car ever snuck up on you?
Lean to the Right: Now lean until your head is hovering over the center console and adjust the passenger's side mirror the same way. You should just barely be able to see a sliver of your own car.
Note that this adjustment routine only works if your mirrors can be adjusted electronically. If you have to move the mirror glass around with your hands rather than with a switch, then you have to have the window open or walk around to the other side of the car. Give the head position your best guess in that case, or enlist a friend to move the mirrors while you lean.
A quick word about the center mirror: Sit normally before you make the adjustment. Don't suddenly develop Marine Corps posture if you're a sloucher, and don't try to show the mirror your good side. Also realize that some days you're a sloucher, and some days you're a ballerina. Simply adjust the mirror for that day, which means when you glance up while driving, you can see as much of the back window as possible.
That's all there is to it. No more swerving wildly into and out of lanes because someone is hiding out in your blind spot.