Theo - you should keep signing up with companies until you have all the work you can do, and then you should sign up with 25 more. One reason for this is that some of the companies you are working for will lose the clients you are shopping, and another reason is that some of the clients you are shopping will end or interrupt their shopping programs. This business changes all the time, every day. Part of your time will always be spent looking for work. You can't make too many connections.
There is a link at the bottom of this page to a list of mystery shopping companies. Sign up with a few every day. Lots of shoppers are signed up with over 100, some over 200.
I don't know where you are, so I don't know what to recommend to you. Here are some reputable national companies: Corporate Research, Maritz, Bare, Market Force, Bestmark. There are many, many other good companies. I have named these because I believe they all have nation wide work and they all pay on the dot.
To get the best feedback, you will have to ask specific questions. Exactly what kind of help do you need?
Mary Davis Nowell. Based close to Fort Worth. Shopping Interstate 20 east and west, Interstate 35 north and south.