I use to do this with the youth teams I coached. In the first practice first thing I would time them to 1B without any other instruction. I would tell them their times as they passed. Then I would instruct them on running form, have them practice in place and I would have them run to first again. Their times always got better (most of the time Legitimately!). We practed running form again, and the times got even better! Running is something that can show immediate results.
I would then talk about the power of practicing and how some changes follow a learning curve. I use the example of the baby learning to walk. "He crawls really fast at some point, and could get around like that just fine, but must learn to walk. As he trys this new technique(walking ) he falls, sometimes alot and sometimes it hurts, but he continues until he accomplishes walking". This way they are ready when set backs happen. Without telling them this they would revert back to their t-Ball swings because "it worked then".
Now with the big dudes in High School, I think knowing your stuff helps them buy in. Of course having had most of them as little dudes helps.