While bowling may not be the most strenuous of sports, it does require specific skills and physical attributes – balance, muscle strength and flexibility in particular. Strength training with weights is probably the best way to build up the muscles used in bowling. Training the leg muscles can help improve balance; training the abdomen and back can increase stability; and training the arms, shoulders and chest can enhance accuracy and power.
If you are not currently strength-training, be sure to start with low amounts of weight and increase it gradually as you become stronger. You should train 2 or 3 days a week, always leaving at least one day of “rest” between workouts so that muscles can repair and build.