Write a miniature resumé. Mine is usually something like, "I am an experienced and reliable Mystery Shopper who is a Gold-certified MSPA member (Cert number 000001). I have performed hundreds of assignments for dozens of Mystery Shopping companies, ranging from phone and retail shops to fine dining, upscale bar integrity, and overnight hospitality assignments at four-star hotels."
Just make it true. Even if you're new, you have some qualifications for this. Say you have access to a laptop and high-speed internet, and you take your commitments seriously. That in itself makes you eligible for plenty of shops.
"The future ain't what it used to be." --Yogi Berra