I agree with Flash. I recently added some new companies and now have more options available to me. It is nice to be able to pick and choose which shops I'd like to schedule for myself instead of having so few options that I need to schedule anything! With these new companies, I am seeing better paying shops than I did when I began mystery shopping. I think it's because I initially signed up with some large companies that have lots of opportunities and are easy for newcomers to work with, but pay poorly.
Besides signing up with more companies, I recommend researching the companies first. Make sure they have a good reputation for paying their shoppers... very important!