I have the same problem. Here are a couple of tricks I have learned. I found the finger shake is magnified if your arms are not braced against something.
If the larger part/assembly is sitting on the bench, I try to rest the heal of the hand holding the tweezers against the workbench surface. If my hand is held in the air, the strain of trying to hold the arm in exactly the right position adds to my finger shake. Resting the tweezer hand on the same surface as the model or assembly you are attaching something to does help.
If you are using two hands- say your left hand is holding the model or a major assembly with your left hand, and your right hand is holding a part with tweezers, the arm problem is magnified. I touch the little fingers of both hands together while I work. This connection reduces the problem only to the positioning of the fingers and greatly reduces the shake.