Your USP is a fantastic weapon. If you craft it carefully, you can wield it like golden handcuffs on unruly, unreasonable, unappreciative and disrespectful clients.
For example, mine is "increase business opportunities by 30% within 180 days or your money back".
+This locks them in, de facto, 180 days
+I only take on clients NOT on Page 1--putting them on Page 1 guarantees AT LEAST a 30% increase (it's all in the math) in traffic which is a unique visitor is contractually defined as a "business opportunity".
Por ejemplo: This client didn't rank for anything and had roughly 10 uniques per month. Putting him on Page 1 for a single 1900 local search term in 10th position statistically will feed 2.2% or 40 uniques...a 400% increase.
+ Your contract should clearly spell out what you do (in terms of benefits, results, advantages--not the tactics/tools), the objectives and why the client is seeking your services and any deviation from YOUR "game plan" voids the guarantee.
Here's the juicy part, this egotistical pain in the @$$ called my account rep demanding we include both locations and phone numbers of his business in a video which would KILL conversions.
I have a policy with my reps, any request that goes against my published monthly best practices (published for clients and employees) is directly forwarded to me if the client insists after the rep corrects them.
This is where the USP comes into play:
"Mr. Client, I will do this after advising you against it, now be advised this will void my guarantee as this is your idea and not my own".
Needless to say he changed his mind. And I reminded him again, that he "knows the rules" as they are published to him monthly and any requested violation will void the guarantee.
I'm a huge proponent of client relations and my published policy to my account staff is "blow the client away"--however you MUST make it clear that your firm are the experts in digital marketing and they are a CLIENT, not an employer.
Carefully positioning yourself as such, and holding your clients accountable for their indiscretions creates brand cache and scarcity, enabling you to charge more and retain clients longer.
That being said, when crafting your USP, state your chief benefit (consistently achievable to you, most desirable possible to them), add a timeline and the word "guarantee" and you just created your biggest sales asset. Ensure all of your personal marketing and branding does not alter, differ or otherwise contradict this statement.
Anyhow, I hope this helps.