Tip #1; Agree to take the assignments that nobody else wants and do an amazing job on them.
Tip #2; Simply put the date you want the assignment for in the text box and let your reputation speak for you.
Unless the scheduler is desperate for a shopper, they are not going to take the time to read a paragraph of text in the application. There are over 1,500 shops offered each month, and many of them which require emailing back and forth with evaluators. If a scheduler worked 5 days per week and read just one paragraph for each assignment offered, that would be over 75 per day they had to sift through. Now imagine 10, 20 or even 50 people allying for the shops and leaving a paragraph.
Keep it as simple as possible!