I used to do it...
I got started because a good friend worked in Job Service in the temp jobs, so she would line up people to do one-time only jobs and a lot of that crowd weren't too reliable, so she would call me when she got in a bind. I mystery shopped at Scheels Sporting Goods store, Panera Breads, a couple other local stores. Each one had different rules. Scheels was HARD because you had to keep track of names of all the employees you saw, who spoke to you as you passed and you had to record nearly your whole conversation with the sales person but it paid a lot, I think $25 per visit. They also would give me items to choose from, eg - guns, in-line skates, etc and they were pretty specific on what questions to ask. I could do as many as I wanted but I think they started to suspect me because I was in there asking a million questions every day! Panera just wanted to know if the workers were courteous, the store was clean and the food was fresh, and they reimbursed me for the cost of my meals so I didn't make any money there per se. It was fun though, I enjoyed it!