When watching a truly accomplished guitarist play, it is always notable how little effort they seem to be making. Movements are small and efficient. But they are also graceful and elegant. This graceful and elegant approach to moving the hand is the key to effective left hand shifting.
When a shift is done incorrectly, it tends to look a bit wooden: there is a jerky quality to the movement and the wrist is held stiffly. Shifts are most efficient when there is a bit of play in the wrist. I regard the movement as sinuous, liquid.
It helps to think of the movement in two parts: the arm moves slightly towards the new position while the fingers remain in place, then, quite suddenly, the hand "snaps" into place in the new position. When moving down the neck from an upper position, the initial movement is led by the back of the wrist, and causes the hand to slightly collapse forward as if flexing (but without using the flexors). If the shift is only two or three frets, then this slight movement positions the hand perfectly, and the hand then simply snaps into place. If it is a large shift, say, 7 frets, then this slight wrist anticipation much reduces the amount of motion. The effect is multiplied if, on landing in the new spot, the hand reaches out a bit to the left, effectively beating the arm to the position by an inch or so (though this effect will be mitigated if the necessary shape on landing doesn't permit it).
When shifting up the neck, say, from the second position to the fifth, then the movement is led by the heal of the hand, specifically by the corner farthest from the thumb (beneath the 4th finger). The wrist tilts the hand back a tiny bit while still holding the note(s), leading up the neck with that lower right corner of the hand. (The hand looks slightly extended). Then the hand "snaps" into place, basically realigning the arm, to complete the shift.
I find it's relatively easy to move the hand extremely fast to a position which is in normal alignment with the arm. Therefore, the goal is to effectively position the arm in the new position (slightly), while still holding the current position. Then the shift is a correction in alignment rather than a leap to a new location. Incorrect shifts look as if they are stiffly led by the fingertips. Correct ones reveal an arm/wrist that is as liquid and flexible as a dancer's.
Independently of the instrument, one can practice the movement. Moving the left arm back and forth horizontally in a space not longer than a foot, palm up, allow the wrist to be loose and floppy. The arm leads, the hand follows after. The sensation is easy, relaxed, pleasant. Not tight, stiff or jerky.
It may be hard to reconcile the two ideas: fantastically complex, difficult activity done with a hand/arm that "loose, floppy, and relaxed"--but this is it. The looser the apparatus, the easier and quicker the movements are. (The same is true, of course, for fast playing--only with loose, relaxed muscles, can you acheive fast finger alternation).
Once the sensation of the loose shift, as described above, is rehearsed and understood, shifts stop being a problem. I find I can move my hand with lightning speed the entire length of the fingerboard, with no special effort. This facility can have a huge impact in your ability to render rhythms precisely, to get the music to sound as desired and not be a victim of unnecessary bumps and interruptions in the rhythmic flow. Further, it is always a net negative to permit tension to accumulate in the hand/wrist/arm, and stiff shifting is a key cause of this problem.
When I was a kid, it was fun to try and imitate the look of the cobra rising out of the basket to the sound of the Indian flute, as represented in so many bad movies. I would compete with my brothers to best get the effect: clasp your hands together, Palm to palm and put only your index fingers outstretched, together. Then wiggle them slowly, side to side, first the tips then the rest of the fingers, in a seductive (and ridiculous) imitation. And yet, it is this very motion that I recognize I'm using with my arm when I shift. Its the writhing of the seductive Indian cobra!!