The Left Hand
A Jolly LH 1st and 2nd fingers
I encourage keeping the 1st and 2nd fingers of the left hand apart as it aids in setting up the desired relationship between the forearm and hand through the wrist.
Once that is established and the player has learnt that the forearm needs to carry the hand then it becomes less necessary to be so conscious of doing so.
Also useful in helping students to learn to stop the LH thumb at the back of the guitar from dictating to the fingers at the front how they should present to the fretboard which is always an issue with beginners.
IE If there are no fingers placed on a note on the fretboard then the thumb should not be on the back of the guitar either.
So, to help them to learn where the thumb should go, I get them to place their finger on a note first (utilising the 1st and 2nd finger trick to establish the desired forearm/wrist/hand anatomical structure and generally the 3rd finger at the 3rd fret on the 4th string)
and then place their thumb and voila! that is where the thumb needs to go!
So the fingers are dictating to the LH thumb rather than the other way around.
This also better equips the student to address the 3rd finger 3rd fret/4th finger 4th fret conundrum as they have immediately added to their ability to cover distance on the guitar as all the extension they require is created between the 1st and 2nd fingers
and the 3rd and 4th are sitting comfortably over their home frets. Helps with big stretches later on also as they naturally spread out their 1st and 2nd fingers to gain distance.
I am a 3rd finger 3rd fret guy on the top strings in 1st position as it encourages students to work hard on the 1st and 2nd finger thing so as they can master it.
Obviously, where the chord/arpeggio/some scale passages require it or a students hand is small, I encourage the use of the 4th finger at the 3rd fret.
A Jolly Hammer On
I teach my students to learn to use the weight and speed of the finger rather than finger strength to make the note sound. Picture the action required to use a hammer to drive in a nail.
The head of the hammer starts close to the head of the nail.
The head of the hammer is drawn back.
The head of the hammer is driven into the head of the nail.
A video demonstration of the technique
A Jolly Pull Off
I teach my students to perform a pull off by using the joint nearest the tip of the finger they are using to make the sound by:
Firstly placing the fingers on both the notes they are intending to perform the pull off on.
Then pulling the finger making the sound quickly off the string until it is resting on the string beneath (closer to the floor) the one on which they are performing the pull off.
A video demonstration of the technique
A Jolly Full Barre
In front of the fretboard
Looking down the fretboard
Barres can take sometime to work out and more often than not it is due to a faulty technique.
Note the position of the thumb, the need to get the wrist below the fretboard in order to get the finger dead straight and that the little finger side of the hand is relaxed and free to be used.
The main muscle I am utilising is the big one between the thumb and 1st finger at the back of the palm rather than the big thumb muscle on the inside of the palm.
I find most beginners try to use that internal muscle which causes the palm to close up as the large muscle on the little finger side of the palm also contracts.
This generally occurs because the thumb is not dropped beneath the bottom curve of the back of the neck but is left up higher and trying to squeeze the neck between the thumb and the 1st finger.
Playing a full barre should feel very different to your normal hand position.
A Jolly playing beyond the XIIth fret
Playing that nasty, nasty little passage in bars 11 - 16 of the A major section of Barrios' Mazurka Appasionata which requires a half barre at the XIVth.
Being able to get around the neck at the top of the guitar does rely on an appropriate sitting position and I have included an image showing the posture as I am playing this passage.
Whilst my thumb remains placed almost on the side of the fretboard, once I am at the XIIth fret, you can see that when I move to the XIVth fret to play the little half barre I release the thumb entirely as both 'shreda and ccary have mentioned one should do.
A video demonstration of the technique