This article was excerpted from Weekend Entrepreneur: 101 Great Ways to Earn Extra Cash, available from Entrepreneur Press.

As a stay-at-home mom, Deva Roberts was "looking for a way to be able to bring home an income while still being able to care for my children. I had just found out I was pregnant with my fourth child and knew we would not be able to easily survive on the modest income my husband brought home. Two children in diapers is not cheap." Roberts has always been interested in human nature and is a natural born "people watcher." She pays close attention to detail and as a mystery shopper and customer service auditor she gets paid to do something she enjoys.

Mystery shopping companies pay shoppers an average of $10 to $30 for completing an assignment. Sometimes they are reimbursed for services, such as oil changes or meals at restaurants. It's a good opportunity for stay-at-home moms like Roberts; her children are 11, 8 and 3 years old with the youngest at 22 months old. Many assignments allow you to bring your children in tow. After all, the average customer has children too. Shoppers need to use caution when applying to mystery shopping companies. "One rule of thumb is never give money to a website that says you need to pay in order to get the names of mystery shopping companies because they are only out to steal from you," Roberts says. "If you stick with companies that belong to the Mystery Shopping Providers Association, you are in good hands as they will not allow a mystery shopping company to be a member if they are not up to their standard."

When Roberts started out, she says the internet was her best friend when searching for information on mystery shopping. She did a Google search for "work from home" opportunities and stumbled across mystery shopping. "I was one of the lucky ones who found legitimate sites for mystery shopping for free," she adds. The more companies a shopper signs up with, the more likely they are to get regular assignments.

"There is no real 'investment' needed at first," Roberts says, "other than to have a computer with online access and a printer." However, you do usually start out using your own money for the mystery shop assignments themselves. Roberts explains you either request or get an assignment with a company you are signed up with. For example, there are clothing shops where you're required to make a purchase and you receive a reimbursement, but you won't get reimbursed for at least one month later. Your payment for the assignment and reimbursements can take anywhere from one to three months, depending on the mystery shopping company for whom the work was performed. Roberts' time is divided between working in the field and filling out reports that have to be submitted before the company she has completed the assignment for will pay her.

After the visit, the mystery shopper goes online to transmit the report to the mystery shopping company, who then edits the report and verifies it for authenticity. Once the editors give the report the final review, results are available online to the managers. Reports are often delivered to the company within 24 hours of the evaluation. Since everything is web-based, businesses are able to view all their reports in one place.

As a stay-at-home mom, Roberts gets paid up to $1,000 a month while she works part-time. She quickly discovered that networking with others in her business and perfecting observational skills is a must if you want to be good at your craft.

For more ideas on businesses you can start on the weekend, read Weekend Entrepreneur: 101 Great Ways to Earn Extra Cash.