Labor experts predict a shortage of workers over the next two decades. The good news: The best jobs will pay well, even though they will require more education. The bad news: The continuing shift to a service and retail economy means more jobs — such as cashiers and clerks — with low pay, few benefits and limited upward mobility. Yet, the middle ground is not barren. Many high-demand jobs will require modest education or training and still pay fairly well. Jobs in nursing, computer support and dental hygiene only require associate's degrees; desktop publishers and tractor-trailer truck drivers need only on-the-job training and a vocational certificate. To succeed in the workplace, experts say tomorrow's workers must continually update their skills. They also warn job seekers to be realistic: High aspirations are fine, but there are only so many good jobs to go around.