The Great Recession changed the American workplace. More and more jobs that used to be full time are now part time, with lower pay and limited or no benefits.

The number of "involuntary" part-time workers—people who want a full-time job but can't find one or have had their hours cut back by their current employer—has nearly doubled since 2007. There were 8.1 million involuntary part-time workers in October, versus 4.3 million in October 2007, according to the latest figures from the Bureau of Labor Statistics.

"Using part-time workers increases flexibility and gives companies an ability to adjust to changing business conditions," said Aparna Mathur, an economist at the American Enterprise Institute in Washington. "Unless we see economic conditions improving, I think this trend will continue."

In addition, the nature of part-time work has changed. At many companies, part-time means "on-call" with no fixed number of hours per week. It's the equivalent of just-in-time inventory for the labor force.

"It's an extremely abusive and unfair practice," said Carrie Gleason, executive director of the Retail Action Project (RAP). "Part-time workers' hours are constantly changing—and not just weekly, but sometimes on a daily basis."

In a report released last year (Discounted Jobs: Home Retailers Sells Workers Short), RAP interviewed 436 retail workers in New York. Twenty percent of those surveyed said they must always or often be available for "call-in" shifts. More than a third said they were sometimes, often or always sent home early from work.

While extreme flexibility might be good for some businesses, it's rough for employees to deal with constant uncertainty. They've basically become day labor and need to call in every day to find out if they have hours.

(Read more: McFail: McDonald's dubious debt advice to employees)

"This has created tremendous instability for the retail workforce," Gleason said. "It's difficult to arrange for child care, to go to school or hold down a second job, which so many part-time workers desperately need."

Always juggling your schedule

Akaisa O'Kieffe, a 21-year-old in Brooklyn, N,Y., wants a full-time job but can get only part-time hours at the Manhattan clothing store where she works.

Some weeks she works three days, some weeks only two. She always has a shift on Wednesday, but the other days float. And the schedule doesn't get posted until Friday.

(Read more: Black Friday arrests at Wal-Mart wage protest)

The last-minute scheduling leaves this single mom with unpredictable paychecks and a constant scramble to find child care.