take regular breaks,

 never work if your thumbs are in pain,

 follow the basic recommendations of the ASHT (American Society of Hand Therapists).

The ASHT adds to that list:

 

Use a neutral grip when holding the device. A neutral grip is when the wrist is straight, not bent in either direction (not strong or weak).

 

If possible, place pillows in your lap and rest arms on pillows or use the device supported on a desk or tabletop. This will allow you to keep your head in a more upright position and therefore decrease neck strain.

 

Sit in an appropriate chair. This is a chair that allows you to put your feet comfortably on the floor and also provides good back support.

 

Switch hands frequently and vary the use of fingers/ digits This will allow the one hand or other fingers/digits to rest and reduce fatigue.

There may be cell phone novelists out there (people who write prolifically from their mobiles), but for most of us a little TLC on the hands will keep us texting longer.