I've strung my racquets about a hundred times so I wanted to offer a cheap service to people in my tennis class at the local junior college. They're all friends and people that I've played aginst in high school. I tell them I can string their racquet for less than anywhere else in town because I really can. It's just like they don't believe I can do a quality job. How can I convince them that I'm not just some crap stringer?