I think you could have done the steelmanning yourself, but here you go:
1) Rewards can be social, in other words virtually free. You don't have to reward all good behaviour, in fact that probably makes it less effective. (like you already said yourself?)
2) Here's how I would do it: Always reward exceptionally good behaviour, sparingly reward ordinarily good behaviour.
This doesn't mean punishment should be never used, but it's difficult to build a positive relationship with someone you're punishing constantly.
sparingly reward ordinarily good behaviour.
Of course, that means that unless you also punish bad behavior, it won't stand out from the ordinary good behavior.
but it's difficult to build a positive relationship with someone you're punishing constantly.
It's also difficult to build a positive relationship with someone who is constantly engaging in bad behavior.
Related: Son of Low Hanging Fruit
Another post on finding low hanging fruit from Gregory Cochran's and Henry Harpending's blog West Hunter.