Hang a pulley on the wall and run a rope through it. Weights on one side and a handle like on a water ski rope on the other. Start light with your bow hand against the wall and draw the handle to your anchor point. You must simulate the draw to get the most out of it. Do reps of 12 and 3-5 sets every other day. I have had guys at the gym benching 400lbs having a tough time with my 70# bow.
Or shoot every other day 30-40 shots cranking up your bow every so often and that will help too.