On the verge of sounding dumb...
How is it me saying here take this money and make it work better, you being beholden to me.
I won't make payment requiring certain milestones. I'm not managing the project. I just saying... here, take my effing money and make this work better.
Even if you improve the thing in imperceptible ways (and YES, I'm a 100% kind of job of that type)... its still worth the friggin money to me.
You gotta remember, I'm all about working better... perhaps not functionality improvements, but work-ability means more to me than most.