The best endorsement you can ask for is from a current client. But the only way to get this recommendation is to make sure all your clients are delighted with your work. Even if a client turns out to be difficult or challenging, focus on making sure the end product is the highest quality you can produce and that you offer service with a smile. You can always turn down future work from this client. Just remember that every client talks to other potential clients, some of whom will be nicer and may have more interesting work and perhaps even bigger budgets for you.
It's also important to remember that your best client is your current client. It takes an awful lot of time—and time is money—to get up to speed with a new client. Make the people you're working with today happy, and they'll keep sending you work. And more work from the same client means you can spend more time designing and billing, and less time prospecting and pounding the pavement.