I'm not a secret shopper, but one of my good friends used to be one. It was pretty cool because she worked for a company that had many clients from restaurants to stores, and management groups, even airlines. Most of them seemed to be restaurants though.
The cool thing for her company (I'm sorry, I don't know the name) was that she almost always had to bring a friend for accountability purposes, which is why I got to go with her a few times. I think her company paid her per report, and they gave her a sort of petty cash fund, or sometimes had to reimburse her.
Some of the companies were looking for very specific things, some just wanted a general overview of the service. Time frames varied. Sometimes she would just have to go between certain days of the week, but sometimes they wanted her to go between a certain time period of a particular day, which was a little more challenging.
She got into it through a friend who already worked there. Apparently that company really liked it if you had a buddy you could partner up with and who would look for the right things.