Build yourself a heavy driver and swing it 30 to 50 times as fast as you can while staying in balance. I pick up an old steel shaft persimmon driver, pulled the grip, filled the shaft with lead shot, added grip tape and installed a new grip. I now have a 3 pound driver to swing. Swinging heavy driver fast will work all the same muscles you use with your normal swing. In a few weeks you'll notice a big improvement is your distance and balance.