I've recently developed a tremor that comes & goes. Sometimes, I shake like a leaf. My neurologist said my tremor has a "postural" component, i.e. it manifests itself most when I'm doing something with my hands, especially fine motor skill stuff. He also suggested wine, as another poster stated.
On the other hand, my handgun shooting is primarily defense-oriented, and I've recently become enamored with point shooting. As taught by Fairbairn & Sykes, Applegate, etc., they emphasize convulsive grip, "aiming" w/o sights, and firing as soon as the gun covers the target. Under these conditions, tremor is a non-issue
When I sight in loads, I shoot off sandbags, which ameliorates the tremor.
With all of the above said, I agree with others that practice will help, if it's not a physical-medical condition, per se. Specifically, practice establishing your "natural point of aim" as you prepare to shoot. If that's new to you, it means take aim, close your eyes briefly, and then open them. If your aim has wandered off target, adjust your stance. Repeat as necessary until the problem is remedied. This is your "natural point of aim" in that stance, with that gun, and it allows you to reduce or eliminate muscle tension caused by having to strain to re-position your gun on the target. Thus, you shake a lot less.