With my boys (at least the upper-level ones), I take a fairly unorthodox approach to this: on conditioning, I allow them to determine the difficulty of their own workout. There's no "cheating" because there's no specific level of intensity they have to do; I tell them I'm completely ok with them making it easier on themselves if they need to.
Generally what I've found is that when I first start doing this with a kid (ie when a kid moves up to the upper-level group for the first time), they go really easy at first. However, after a few practices of doing the bare minimum, they realize several things. First, they're not winning any "victory" over me by avoiding something I want them to do (which is important with the 11-14 age group, where the attitude kicks in); 2) they finish before everybody else and then have nothing to do but sit around and wait for the rest of the group to catch up; 3) while they're being lazy, everybody else is getting a lot stronger; 4) The girls team is watching, and nobody wants to look like the weakest team member in front of them.
P.S. This is L7-L10 (the highest levels at our gym)
It usually takes a week or two, but after that they really push themselves without me having to do anything to encourage them. I don't have to push them; they push themselves.
There's also an element of reverse psychology at work; the more I tell them "it's ok, you can make it easy on yourself," the more they want to challenge themselves.
What I love about this is that it really leads them to be internally motivated. They don't push themselves to keep me happy, they do it because it makes them happy. They know they're getting stronger, and once you strip away the distraction of a yelling coach, that's all the motivation they need.