Kinda what John said. Draw your bow slowly (3-4 count) as if actually making a shot. Come to anchor, get back tension, concentrate and hold for 3-4, then slowly let the bow relax for 3-4 count. Try for 3 sets of 5 or more reps. Repeat as many times as possible from the opposite side to balance your strength growth left to right. This balance is important to your skeletal structure. Grip exercises and forward and reverse wrist curls help a lot with release and bow arm control, too.
If you go at this kind of thing kinda hard, be sure to give your muscles a day or two of rest between sessions.
Those spring or rubber band exercise thingys can be a big help, too.