The key to holding a bow steady is maximum relaxation. If you flex a muscle, it will shake; if you relax a muscle, it will be steady. Obviously, you can't relax completely, but you can relax most of your muscles by using the skeletal system as much as possible, because bones are steadier than muscle.
You want to use the biggest and strongest muscles -- the ones closest to the spine -- to hold the bow up and back, because these muscles are steadier than your arm muscles. Remember, the farther away from the spine and the smaller the muscle, the less steady it is.
Drop your bow shoulder, relax your arm, and let it hang from the shoulder bones. You need to relax the bicep and forearm of the release hand and pretend the elbow is doing all the work. This uses the big muscle between the shoulder blades to hold the bow back and up. Once you reach full draw, focus on relaxation, and your bow will steady itself.
Lastly, if you're shooting a bow with a draw length that is too long, you'll never be steady. Shorten up your bow's draw length and you'll feel much more relaxed at full draw.