For my two cents at least, there's two main principles that seem to be working very well for me in my own business. I don't need a ton of clients though, and I'm building a personal brand, so once you're working with sales people the equation might be very different.
But for me at least, the two things that seem to matter, are finding the right kinds of business owners, and starting with education. Find business owners that understand the difference between an expense and an investment, owners that have clear goals for their business and are ready to put in the work to make it happen, and owners that are ready and willing to invest the time needed to follow your directives (gathering reviews, working with you where necessary for backlink building, making themselves available for citation phone verification when necessary, taking accurate records about new clients and how they found the business and sharing it with you, etc.)
Once you know those owners, the easiest thing to do is just to share the power of what this stuff can do for their business. Give them knowledge they can use to make an informed decision, and (ultimately) the right clients will realize they'd rather bring in a trustworthy, knowledgeable expert to take the reigns in that area of the business. Even a single webinar can easily accomplish all that if you fill the seats with the right people. A salesman could help, but I feel like even for a bigger company, you're much better off using salesmen to sign up hot leads at the end of your funnel, or for filling seats at the beginning of your funnel, than you are having a salesman try and find and convert cold leads all on their own. Otherwise you're relying on promises, I'd much rather have a business owner just grateful they found someone they know is competent and that has their best interests in mind.
I do believe it's important to be results minded, and to make sure you're being paid from a percentage of new revenue (in the long run) but I'd much rather be guiding that conversation, instead of making direct promises. I'd rather have a conversation around what their goals are for their own business, how many new clients they're looking for, what their ideal clients look like, and then stay involved with them and make that a mutual goal you work towards together. Providing ranking reports in my experience at least hasn't impressed very many clients so far, I still include a bit of lip service there, but I haven't met any small business owners that actually care about it if that's what you focus on, and worse, if that's where you focus, you're much more likely to end up with clients upset that you aren't in one particular 3-pack they've decided is important, regardless of how big of a difference you've made in their bottom line.
To really answer your question though... after talking with a few business owners in your niche, you'll get a sense for how they measure success. For wedding photographers, it's about how many bookings they're getting per season, and how much they can charge per booking while still getting new clients. I wouldn't promise them specific numbers when signing up, but I do work hard to actually reach those numbers, because those are the kinds of stories I love using as testimonials. Working with clients to track and organize those statistics doesn't just make it easy to keep them as a client, it also makes it easier to find new people next time.
