offering services for free at first

when starting out, i’ve heard that an effective approach is to offer your services for free in exchange for testimonials. in my case, it’ll be a weightloss program, but i also really understand when brooke explains that the cost of your service will make a big difference in how your clients show up. how do you get this same effect with your clients at the start who aren’t paying? or would you recommend that it not be free?