Split ends can be caused by repeated blow-drying, heat-straightening, or excessive brushing. Once your ends are split, the only solution is to go to the salon to get them trimmed.
You can avoid split ends by brushing less, using less heat on your hair, regularly using conditioner, and protecting your hair from extremely hot or cold weather.
Regular haircuts and trims are a good idea, too. They help rid your hair of damage so that a healthier crop can grow up top.