Beginners' Primer

Practice Tips

1. Learn the necessary chord changes before tackling a piece of music.
Although it may seem tedious sitting down and merely moving from C to G to Em (or whatever the piece demands) over and over again, this is the quickest way of conquering any composition. By focusing first on just the movements required by the fretting hand, you can apply more concentration later on to the strumming/picking hand when it comes to learning the rhythm.

2. Don't practice til you get it right. Practice til you don't get it wrong.
Training your fingers' muscle memory requires constant repetition. Getting it right one time won't make it. You need to be able to play it accurately every time.

3. Playing barre chords.
The trick to playing 'E-form' barre chords properly is keeping the first (index) finger straight all the way down to the third knuckle joint. Watch out for curvature in your index finger (which may cause muting on some strings) and keep this finger as parallel with the fret as possible. Pay particular attention to sounding the G string clearly as this string bears the only 3rd interval in the chord and muffling this note will yield a R-5 chord instead.
When playing the 'A-major-form' barre chord I recommend using the 3rd or 4th fingers to barre across the D-G-B strings. Most importantly, the first joint of your 3rd or 4th finger should be flattened whilst the second joint should be bent which should cause your hand to fold comfortably around the neck. Keep your thumb on top or hanging over the shoulder of the neck, try to keep your muscles relaxed, and avoid pulling your wrist downwards as you would if you were playing an E-form barre chord.
You may find when playing the 'A-minor-form' barre chord that the wrist of the fretting hand pulls towards the headstock. This can cause the 4th fretting finger to straighten, muting the B string in the process. The B string carries the 3rd interval in the chord and muting this note will yield a R-5 chord. To correct this, tuck your fretting hand palm in towards you so that curvature of the 4th finger is reintroduced. This chord shape should be played with the same technique used for playing E-form barre chords (wrist bent, index finger straight).