Fitness balls will help strengthen your core muscles, and they are fun to use. Simply sitting on a fitness ball while watching TV engages your abdominal and lower back muscles as they work to keep you from falling off the ball. But you'll reap greater benefits with some basic ball exercises. Here's my favorite: the basic fitness ball sit-up:
Start by sitting on the ball and roll back so the ball is centered under your shoulder blades and your head is dropped back towards the floor behind you. Place your hands on your ears and curl up into a sit-up. Release. Start by repeating 10 times and work up to 30. If you are new to fitness balls, keep your legs wide (3-4 feet) as you sit on the ball. Once that feels easy, move your feet closer together so they are shoulder-width apart; this makes the sit-up more challenging.