Dallas Kemp, one of the studio painters at Privateer Press (makers of Hordes/Warmachine), offers some basic advice on how to save your neck, back, and hands when painting gaming miniatures. This could also apply to any sort of small-object, close-up painting work.
The main points to keep in mind are:
Sit Up Straight!
Sit up straight (in a good chair). Elbows on the table (take that, mom!). Heels of the hands together. Now, hold the object in one hand and your brush in the other. Your hands in this position help reduce muscle fatigue and steady your painting hand.
Use a Painting Handle
If you do a lot of miniature painting, holding the mini itself can cramp your hand. It’s better all-around if you mount your miniature to a painting handle. These are easily made (Dallas made his from a chunk of dowel and a 55mm gaming base) or you can find 3D printable ones on Thingiverse. The idea is to grab onto this vs. the pinching grip action of holding the mini itself.
What Good Painting Posture Looks Like
With all of these tips in place (let’s add really good lighting to that), you should be able to paint for hours with minimal stress and strain.