I'm going to angle off in a slightly different direction. (Hey guys, been away for a few months)
1: Pay attention to your posture; that you are sitting up straight. This has an influence on your breathing.
2: Be sure that you get a comfortable chair that you can sit in for a while.
3: The angle at which your right( if you are right handed) arm comes over the face of the guitar is kind of different for hollow-body vs electric (slab) guitar. I believe that acoustic guitar "delivers" your arm to the face of the instrument at a superior angle. Playing an acoustic tends to expand your arms away from your sides, and this gets reflected in your left (fingering) hand, symmetrically. I realize that an acoustic guitar is harder to play and harder to play for longer periods of time. If you are going to practice on a solid-body, no problem, but try not to suck your arms into your sides; Consciously try to keep your arms away from your sides.
4: I have always found that hand tension ends up being matched left-hand-to-right-hand. If I am gripping the neck really hard, for example, because the action on my guitar is really high, I find I am clenching my right (picking) hand. If I am gripping the pick really hard, I find I am gripping the neck really hard with the other hand.
5: Observe the difference in left hand position between a rock player (typically thumb wrapped around the neck) and a classical player (left hand forms a "U" shape, and the thumb is nearly parallel to the fingering fingers. The pad of the thumb rides up and down on the highest point of the arc of the backside of the neck, right down the spine. These are two utterly different approaches to how you hold your left hand on the neck. I prefer the more classical approach. Others may not. I think that wrapping your hand all the way around the neck slows things down; but I have also seen very fast players do the big wrap. It's great for power chords, but IMO it is bad for single string. I think my point is that there should be a "model" or "idealized" position you have in mind for your hands that you try to adhere to; and you don't just let things fall where they may, however they may. Your posture, your arms, the way you hold the neck & the pick....these things are "one way" and you try to maintain that way, consistently.
6: The single most efficient way of practicing I have ever encountered is: Need a clock, a metronome, an amp with headphones with slight reverb. You play ONE SCALE. It can be any scale, but it might as well be a useful one. Play that ONE SCALE in eighth notes against the metronome for 5 minutes, up > down > up > down.
5 minutes. No changing. No riffing. No improvising. No deviation. Don't slide into notes, try to make every note exactly the same. Don't bend notes. Start very slow. During that time, you are watching your fingers and trying to make them like a machine, like the rocker-arms in a pushrod engine, and trying to exert less and less force on the strings, just enough to fret the note, paying attention to how close to the fret you are fretting the note. On your picking hand, you are just watching it, looking for inefficiencies and wasted motions. After 5 minutes your left hand will be absolutely on fire. But in one shot, you get serious metronome practice, scale practice, posture and hand position practice.
Above all: Practice with a metronome or rhythm machine. I can tell a player who has practiced with a metronome vs one who hasn't in about 2 measures.