Plucking the string can be caused by not coming to full draw, where your drawing forearm is in line with the arrow as viewed from above, or by losing back tension at full draw, where your drawing hand creeps forward at full draw. You should feel like you're always pulling during the draw, even when you're at full draw and the arrow is not moving anymore. Your hand should move backward after releasing the arrow.
A Formaster is useful in helping you to gain and maintain back tension throughout the shot.
Sometimes, a deep hook on the string will lead to plucking. If you can keep control of the arrow without cupping your hand, you might try moving the string out to where it is between the tip and first joint of your fingers.