Of the 2,407 U.S. companies surveyed, 41 percent said they had difficulty filling these positions. Other jobs that made the top 10 list include truck drivers, accountants, technicians, and machine operators.

Here's a look at the training required to pursue the top three.

Closing the sale

Sales representatives topped the most in-demand list for the second year in a row as the hardest job for U.S. companies to fill.

"It's a supply and demand issue," says Melanie Holmes, vice president for corporate affairs at Manpower. The 2004 U.S. Bureau of Labor Statistics predicted that within the next decade, the U.S. would need an additional 736,000 people on the retail floor and an additional 187,000 business-to-business sales representatives, she says.

The sales representative job spans the spectrum of low paying retail positions to very profitable corporate ranks, such as pharmaceutical sales reps. The job may involve irregular hours and heavy travel. Salaries range from $30,270 to more than $114,540 a year, since earnings are often based on commissions.

Aspiring sales reps should also have the technical familiarity with the product, as well as the personality traits–including patience, persistence, and charm–to successfully engage clients and make the sale.

Employers are increasingly looking for sales representatives with a college degree. Courses in marketing, economics, communication and foreign languages may prove helpful. Many companies offer one- to two-year formal training programs. Seminars offering lessons in sales techniques are increasingly popular. More than 7 million people have signed up for a Dale Carnegie course, one of the more well-known leaders in developing interpersonal skills and building rapport. Enrollment in the Sales Advantage Course, which is designed to help sales reps improve their communication skills, has increased 35 percent over the past year, according to Regina Acosta, manager to the inside sales team at Dale Carnegie. "We build sales people and therefore we build our business," she says.

Certification is available by taking a course and passing exams administered by industry organizations. The Certified Sales Professional seminar, organized by the Manufacturers' Representatives Educational Research Foundation, is a three-day intensive course followed by written and oral exams, aimed at mid-level managers.

Popular sales training programs include ones specialized to pharmaceutical sales, medical software, and technology.

Educating the next generation

Teachers jumped to slot number two on Manpower's most in-demand jobs list. The ranking refers to K-12 teachers, excluding special education teachers.

During the next 10 years, the U.S. will add 524,000 post-secondary teachers and 265,000 elementary school teachers, according to Bureau of Labor Statistics. The National Education Association estimates that close to 2 million teachers will be hired.

"The population is exploding," says Jennifer Smolka, program coordinator for Western Governors University's (WGU) online teachers college. "We're also finding more and more that smaller classes are better, so we need more teachers." Retiring baby boomers are also a factor. "There are a large number of lifelong teachers, and those teachers are now retiring," she says.

In addition to a four-year bachelor degree program in education, schools such as WGU also offers a master's in education for those who already have a degree but need education training. Such master's programs average 18 months to two years, but students who can devote themselves to full-time study can accelerate the degree, finishing in as little as six months.

"Teaching is one of the most rewarding professions–you touch the future and create the adults of tomorrow," Smolka says. "But it's not for everyone. It's very demanding."

Mechanics do well – no matter the economy

With 32,000 auto services and diesel openings annually, according to the National Automobiles Dealers Association, automotive service technicians are increasingly in demand. Through 2010, the organization estimates an additional 6,000 jobs per year. The increase in demand can be attributed to retirement of baby boomers and fewer workers willing to replace them as well as the number of cars on the road.

Many high schools and colleges offer two-year associate degrees where students work with an assigned shop and are trained under a seasoned mechanic. The Accrediting Commission of Career Schools and Colleges of Technology (ACCSCT) and the National Institute for Automotive Service Excellence (ASE) currently certify a number of automotive programs. In fact, ASE has become a standard credential for those pursuing automotive careers. Additional certifications are available in electrical systems, engine repair, brake systems, suspension and steering, and heating and air-conditioning. For those wishing to attain master automobile technician status, certification in all eight areas is required.