I have been mystery shopping about 5 months, and I have probably done about 100 jobs. I currently work for 2 companies, but I have probably applied to about a dozen or so.
I have been constantly doing research and reading every article possible to learn as much as I could to keep jobs. I am very happy working with the companies and would like to say that working in a timely manner, and reporting will get the frequent jobs. Being able to accept last minute jobs at a distance will also help greatly. Paying close attention and remembering events are also helpful. The only negative experiences were with my photos when I first started. Since then I have learned to master my camera and get the best shots I can.