Harvard economist Lawrence Katz and a team of researchers studied worker-training programs to identify traits that align with enrollees' success.

They searched for programs that propel a trainee down a path to 25 to 40% more income for at least five to 10 years, in an expanding field.

A successful training program is all about trajectory. Yes, of course, many programs will indeed snag you a better-paying job, but most only connect trainees with jobs that pay 5 to 10% more for one to two years.

Katz's must-have features for those signing up for training:

Certifiable skills. The first rule of thumb is to seek a sector-specific certification, degree or license. Good examples include a Microsoft or Cisco certification in technology, or a diesel repair license. Don't get a diploma in generic computer or mechanic skills.

Wraparound services. Programs that truly change your life are typically six to 24 months long. Trainees drop out for all sorts of reasons, like child care and financial stresses. Aim for a program that offers a variety of resources that will allow you to complete the program when things get tough. Think on-site child care, financial resources and available advisers.

A relationship with a caseworker or teacher. Relationships are everything. Call the program, and talk to a caseworker or teacher to see if this person is likely to remain available and engaged. "A close connection seems to be very valuable for people who have been dislocated or have had trouble in traditional education," Katz says.

Many employers. You want a program that leads to a number of organizations seeking to hire workers with your new skill set. "So it's not only about one employer, who then has a lot of power over you," Katz says. The presence of many would-be employers also indicates that your new skill set is indeed in high demand.

Soft skills training. Without communication and social skills to succeed on the job, people often fail in the workplace. Make sure your program covers this within its curriculum.

Post-employment involvement. Katz found that successful programs tend to remain involved with both employer and employee after the trainee has completed the program, helping with matters like miscommunication and discrimination, as well as partnering with employers on future job development.

Free, low-cost or income-share payment plans. Cost varies widely by field, but expect to pay in the range of $4,000 to $10,000 for a program that is at least six months long. A good option is a program funded by philanthropists, with trainees who pay back the cost as a percentage of their income.

External evaluation. Look for partnerships with universities and, if not that, any data indicating success for trainees five to 10 years down the line — programs usually happily advertise good results on their websites.