For eating, I made healthy foods available that they liked. This worked really well. One son has two conditions that can land a child in in-house hospital treatment for aversion to eating and he never had issues like that.
For trying new foods, they had to try it, not finish it. I never made them do things like clean their plate. Instead, I planned ahead for how much I expected people to eat at the meal.
I always had a good reason for what I was asking and it very rarely came to "because I said so." On the rare occasions where it did hit that point, they went along with it because it was rare, so they were convinced I had a good reason even if I couldn't get them to understand it.
I have one major rule - if I give an absolute command, it must be obeyed immediately without question. In exchange, I promise to do it very rarely and explain my reasons as the earliest possible convenience. This has helped in situations where it wasn’t practical for me to physically drag them - get out of the way of the car, come towards me NOW and away from that crumbling cliff. Stuff like that.
Most other things fall into place just by showing by example. Show interest in their homework, talk excitedly about science, etc, they do their best in school.
As a parent of teenagers... omg, yes. LOL. I think it's as much because they're teenagers preparing to leave the nest as much as anything.
That doesn't mean the teen years didn't have some bumps. The worst of that was cleared up by explaining to my children that they were being flooded with hormones and a side effect of high levels of testosterone is aggression and anger.
So if they were feeling very, very angry, it wasn't because they were being subjected to extreme injustice they had never before seen. It was because they were hormonal.
That conversation ended with the punchline "You're problem is called testosterone, not my bitch mother." which caused everyone to crack up.
After that I occasionally would say "You sound like an icky, hormone soaked teenager." And my son would say "I am an icky hormone soaked teenager" and he would try to tone down whatever made me say that and I would try to cut him some slack for being hormone soaked and not entirely in control of his big, fat mouth because of it.
Different kinds of families, I guess =)
Did you ever deal with an instance where your kids started telling you "no" in response to demands you felt were reasonable, twisting your original explanations to use against you, for things like eating, cleaning up or hygiene? How did you reconcile the difference?