Your hands may be too shaky for this technique, but I'm prone to very shaky hands myself. (although not for any medical reason)
First, firmly plant the elbow of the arm that's holding the model onto the table, holding the mini in your fingers, with the hand palm-up.
Next, pick your brush up in your painting/writing hand, palm-down.
Place the bottoms of your palms (I mean your thumb/wrist area) together, and hold them there with as much pressure as is comfortable. Your hands should be able to line up if you're not holding anything.
There should be 3~4 inches of space between your hands (depending on size) at the part of your palm that joins your fingers. This should make *about* a 75 degree angle from wrist to finger-base.
This is AMAZING for stability, my painting went from average to contest winning in a few months when I discovered this. I would also suggest people with shaky hands thin their paint a little extra.
Now, as I saw, my hand-shaking isn't medical, so
YMMV, but I hope it helps you as much as it helped me.
*EDIT*
Space marine terminator I painted using this technique. I should have thinned the highlights more, but this is what someone with chronic lifelong hand shaking can do with this stabilising trick.