Healthy practice
As slide guitarists, we are upper body athletes, and need to treat our practice sessions as seriously as any other athlete regards their training sessions. Prevention is far better than cure!
Warm up properly (check out the British Association for Performing Arts Medicine warmup exercises HERE)
Drink plenty of water whilst practising
Do stretches after the session.
Stand up to practise sometimes. Not only does it prepare you better for gigs, but it also helps to counteract poor seated posture.
If you are having problems (eg with RSI, tendonitis etc), you might want to check out the Alexander Technique, or the Feldenkrais Method.