I have the same problem sometimes but only with my right hand. Just so happens I'm right handed.
So here's what I did:
Blu-tack or model clips to make the model as unmoveable as possible.
Hand: Double hand technique. Make a fist with your left hand and place it on the table in front of the mini. Your right hand, place the upper wrist/bottom side of your palm (opposit side of your thumb) onto the fist of your left hand. The left hand provides a firm foundation. With your right hand pressing down, it helps control the shaking. (If you are left handed, obviously, just flip the directions).
Only other thing I can add, take frequent breaks and shake out your hand. Let it rest in an open position for a few before going back at it. Good luck!