You may wish to review Bar Chord Basics before continuing with this lesson.

Self taught guitarists frequently contact me when they get to a point where they can no longer get better on their own.  The most common specific problem is difficulty with Bar Chords.  Either they can't play them without buzzing or their left hand quickly fatigues.  Usually, it's a combination of both.  Working harder is not the solution.  Working more efficiently is. 

Here is the secret:

Use torque to depress the the first fingers
against the strings instead of muscular tension.  In other words, twist - don't squeeze.  Looking down on your left hand as you make a Bar Chord, you'll want to turn your thumb and first finger counter-clockwise around an imaginary axis between them.

Before:

After:

This will put the strings a little more under the side of your first finger.  It is possible to squeeze and torque at the same time, so be careful to monitor left hand for excess tension.  A good way to do this is to practice Barring on
your right arm, as if it were the neck of the guitar.  It will give you instant feedback as to whether you are squeezing or not.

One more tip regarding Barring:

Though hyper-extension is nothing but bad for athletes and musicians alike, it is required when Barring.  Lock the tip-joint and mid-joint of your index finger in their fully extended positions like so:

Notice the parallel position of the thumb and index finger.  This position remains the same while torquing the neck.