I have been doing mystery shops and audits for over 9 years now, and I can honestly say that it’s a fun hobby and has, at times, been an EXCELLENT supplemental income.
Basically, when you mystery shop, you are responsible for taking note of several details about a location and transactions that occur at an assigned place. The company might tell you the time and date to go, what you should purchase (or inquire about), etc. You take note of whatever the client is wanting to know – descriptions of employees, timing of things during the visit, pricing, if certain things were mentioned or asked, etc.
There are many companies that pay you a fee per shop AND reimburse you for an applicable purchase. Some just pay you for the shop and do not require a purchase. Still, others just reimburse you for a required purchase and don’t pay a fee on top of that.
When my youngest was a baby, I was able to reliably make between $200-$600 per month in fees PLUS reimbursements (meals, games of bowling, gas, clothes, etc). One downside to mystery shopping is that the payment can take between 30-90 days to process (depending on the company). That means that while you may “shop” a place on April 1st, you might not get paid until May 30th, for example. It all just depends on how they do payroll.
The #1 resource that I give to people that are interested in become a mystery shopper (or secret shopper) is Volition. They have a HUGE list of mystery shopping companies and all are free to sign up with. DO NOT EVER pay to be a mystery shopper! About 99% of those companies are SCAMS. You CAN get the information they provide for FREE on your own using Volition and the individual company websites!!!
My advice is just to sign up for as many companies as you can. It’s usually impossible to tell which companies are shopping which geographic areas and which businesses until you’re “in the system” with them. I receive between 10 and 20 emails a day from the companies I’ve signed up with letting me know of opportunities to mystery shop. Some of them allow you to automatically assign shops to yourself and others make you “apply” for each particular job. Some companies/shops require that you take a test before getting shops – to make sure you understand their company-specific rules and guidelines.
I highly recommend the Volition forums for more info on mystery shopping. Also, if you want an idea of some of the companies scheduling shops in your area, you might try the Job Search tool here to see what’s open in your area. This is NOT an inclusive list, but it’s a start! You may also be able to learn more about becoming a merchandiser, product demonstrator, or an auditor using these resources. All are great ways to make flexible part time income!
What kinds of questions do you have about mystery shopping?

I did mystery shopping when we lived in Texas. I think I might start doing it again. I had a great time; didn’t make much money, but there weren’t that many places where I lived. I might be able to do some in Lubbock now though.