This is also great parenting advice. Give your kid the feeling of having chosen to eat their vegetables, and they'll devour an entire plate of them.
Only he decides by choosing between 2-3 options we've selected in advance to match what we want the outcome to be.
Similarly it's always bedtime before his actual intended bedtime so he can negotiate some extra time. It avoids so many conflicts when he smugly believes he has gotten exactly what he wanted out of us.
I think works on pretty much anyone, you can just be more blatant about it with some people than others.
I don't like letting him "get one over" on bedtime. When he's old enough to know the difference, then what? You'll have to take away those wins. Same goes for basically everything else he'll want from you.
At least he'll (hopefully) negotiate well!
At the same time, there are always things you can negotiate on. E.g. I don't care if he's up until 10pm if he can prove to me that he will consistently get up on time and do well at school despite staying up late.
We're already playing that with him: We point out in the mornings if we notice he's tired that he seemed to have gone to bed too late, and we "trade" going to bed earlier on school nights against staying up later during the weekend. As a result he sometimes tells us he wants to go to be before the agreed bedtime.
And I'm willing to be flexible about all kinds of things as long as he's giving us something reasonable in return. E.g. he gets weekly home work, and so I'm willing to skip on doing it one evening if I can trust that he will stick to an agreement of more than making up for it the following evening (and if he doesn't, he knows the consequence is much less freedom for a long time).
I've no doubt I'll still have plenty of challenges down the road.... And he is already a master negotiator, though sometimes he's trying to be too wily for his own good and sometimes ends up talking himself out of a good deal because he doesn't yet understand all the options well enough. So he gets maths lessons this way too...
We're careful to emphasize the sanctity of personal choice wherever we can, and to encourage her to make her own decisions based on an evaluation of her options (really, we're fine with her picking any of the menu options we give her; if she picks a gross combination it's hers to eat, not ours). The important thing seems to be that once she makes a decision, she becomes invested in it and is much more likely to see it through.
I probably shouldn't discount the preparation aspect, however. She eats much more of her lunch when she helps to prepare it (and thus knows what to expect when she opens it at school). Similarly, we have a countdown for bed ('bedtime in t-5 minutes! Bedtime in t-2 minutes!') that makes the transition to nighttime more or less painless.
A trivial example would be that if we're eating something that is traditionally a finger food and she wants to use a fork. We'll respond "You can use your right hand or left hand, it's up to you." and it avoids the confrontation completely because she gets to decide. She chooses a hand and life goes on.
Another example is sitting in her chair at the table. She'll normally ignore me when I ask her to climb in her chair but the moment I put a decision before her of getting in the chair on her own or I'll do it for her she hops right up.
It becomes second nature after a while and for us it's been a lot less stress and a time saver.