I have mystery shopped for several years. The key is to sign up for a lot of companies. There are, literally, hundreds - and a company you shop with frequently could be with any one of them!

Here's my site, a good website to get started (of course it is free and informational):
http://www.mysteryshopsmart.com/

If you use Volition.com and MSPA, there is no reason to ever pay for membership. Volition.com has a list of many companies who hire mystery shoppers and all are free. MSPA also lists jobs for shoppers. After I signed up with several companies from the Volition.com list, I was always as busy as I wanted to be. Those sites and companies that charge have just found an easy way to make some money and are taking advantage of shoppers who don't know about the other two sites.

It is hard to make money by shopping unless you plan it out carefully. When I start shopping again, I'm going to try to get shops that I can do when I'm going to be in a certain area anyway. I'm also going to chose companies that pay a fair amount to shop. Some of them pay so little that it's not worth shopping for them unless you're going by the place needing to be shopped anyway. Others actually pay pretty good, though.

The companies that charge are usually (but not always) just informational. You don't need to use those to be successful. MSPA.org has a great forum and a job board as well with enough information to get started. I have NEVER paid to mystery shop (I mean other than spending money that's reimbursed at the actual place of business).