My own practice has changed in that I try to play standing up with a strap whenever possible, which permits me to move and avoid any fixed, tense position. Instead of the neck extending out parallel to the floor, I minimize my left-hand contortions by angling the neck up at about 45 degrees from the horizontal. One injurious habit I'm finding hard to break is holding my left shoulder up when I play. But it can be very hard on the thumb, and letting the neck fall into the web between the thumb and fingers instead should at least be considered as an optional change of pace to rest the thumb. Changes to the instrument may help avoid injury.
Lighter strings are an obvious method to reduce strain on the hands.
This will likely alter your tone and may require a change in your playing style or an adjustment in your instrument setup. Along the same lines, tuning down a half or whole step not only reduces string tension further but opens up new tonal possibilities. Using a capo restores the concert pitch of a guitar detuned in this way, but in addition it shortens the effective scale of the guitar to minimize left hand stretches. Though it may sound unthinkable to the acoustic purist, solid-body electric guitars offer advantages in shape and easy playing action, and with sophisticated electronic processing can provide usable RacousticS tone. Chet Atkins and Joni Mitchell are two acoustic guitar masters who are using solid-body guitars in concert venues.