I was wondering if there are some really clear sites, videos or texts explaining the best practices regarding the way to hold your guitar, both to be efficient and to avoid straining your fingers/arm.
In my own personal experience its a good idea to think about economy of movement; don't worry about hand or wrist position too much; worry about finger position; that in mind i recommend practising with a 'finger per fret' where possible; what I mean by this is if you are playing in a specific area of the neck; position your hand over four frets so that each finger is responsible for fretting notes on its fret. Many scales and chords naturally fall into a four fret block of the fretboard, as deviations are often only a note or two outside of this, its a good place to start.
It is necessary to deviate from this pattern for a lot of things, however if you stick to this and move your hand around the neck with this in mind you will find it less stressful on your body; and your hand and wrist will settle into a position naturally instead of being forced into a position.
You always want to have as little bend in your wrist as possible, if that helps. Less bend also contributes to better efficiency. The height of your guitar contributes to this as well. The lower on your body your guitar is the more you will have to bend your wrist.
Here is a site explaining a couple of hand positions, focusing on the "Classical" position: Left Hand Technique
Here are some images from Fernando Sor's Méthode Complète pour la Guitare, illustrating the standard opposite the fingers thumb position as well as the Hendrix thumb over the top style.
So, it's Classically Approved! Somebody tells you don't use your thumb like that, you say: "Go Read The Sor Method and School Yo-self!"
Any beginner's guitar book or web site will begin with a picture of the right way to hold a guitar.
For the left hand, the important thing is that you can fret notes on every string, with your finger coming down vertically, and without straining your wrist.
For this to happen (for most people) you need your thumb on the back of the neck, and you need the neck facing away from you so that you can do this without bending the wrist too much.
In this position, you can't really see the fretboard, so you need to get in the habit of tilting it towards you when you need a peek, but remembering to tilt it away again afterwards.
There are plenty of people who flaunt convention in this respect though; Hendrix had big enough hands he didn't need to, and would use his thumb to fret the bottom E.