When I started to think of needing to change up my clients as needing to "fire" a client, it really changed my business. At the time I was kind of letting clients walk all over me. I didn't have any established protocols of my own and was still feeling like very much a novice in the field. Realizing that I could fire a client meant also realizing that I had power! That I wasn't an employee, but an owner of a service! It gave me the change in mind-set I needed.
Flash forward to now and I still use that language to ensure I am in the right head-space when thinking about things and then when discussing next steps with a client that isn't right. I also use the idea that a client needs to be 'released' ... basically to go on to someone that is better for them.