If you're low on cash and have some spare time, mystery shopping may be the gig for you. For some, mystery shopping comes very easily, for others, not so much. You need to have a very keen sense of detail, and good grammar skills are a must. You also must excel in communication. Oh, and you need a lot of patience. I've had some not-so-good experiences with schedulers...
I would highly recommend volition.com as the premier mystery shopping site.
I would also recommend reading through the following articles before beginning your mystery shopping journey: