Mystery Shopping (or Secret Shopping) involves going to a business on behalf of the company to evaluate their customer service. Most companies secret shop their own businesses to see how things operate on a daily basis from a customer's point of view. Assignments are available for all types of businesses, including restaurants, banks, clothing stores, car maintenance, and many others. Most of these companies use a scheduling company to screen shoppers, fulfill assignments and the like. If you want to mystery shop, you need to register with scheduling companies that do this.
Below is a small portion of the companies that I am registered with. It does not cost to register with a mystery shopping company. Fill out the information and as assignments become available they will contact you. Most programs will reimburse you for a particular purchase and/or give you a small fee for completing the shop.
If you really want to mystery shop in a big way, I would suggest joining MoneyandShopping.com. I joined this site when I first started. They have over 100 companies that you can register for. They also have many training tools available for writing reports, doing digital camera shops, advanced notice of upcoming assignments and much more. This service does cost about $30 per year but it is well worth it. You can also upgrade to their lifetime membership for $67. If you are interested, click here.
Mystery Shopping Companies These are free to sign up