It's important that educate and inform clients of the benefits of working with a referral agency to maintain loyalty. If they understand why they are working with an agency, they'll be less inclined to leave when they're unsatisfied. Instead, they are more likely to communicate with you and give you an opportunity to address their issues because they trust that you have their interest at heart.
It is also important to share your values with regards to cleaning professionals. If you respect the cleaners, and you convey this to your clients, you will attract the type of clients who respect cleaners too. This creates a harmonious working community and reduces "difficult client" headaches.
Here's a video I did for my cleaning referral agency. Take a look.
*The Agency offers both flat fee & hourly rates now.