Well, I wasn't planning to write a book, but that's how it ended up. I go into a lot of what happens at my own leagues. Take this post from the point of view of a league leader advising other league leaders on how to improve their kids.
First, I agree with all the above posters. World Championship decks are a great way to demonstrate how quality decks are built, and they can be picked up cheaply.
Next, understand that some kids will never really progress to the point of competition. They just want to have fun and play what they want, and that's just fine. That's the whole point of league -- to have fun. Don't try to force it into being ultra-competitive. At any league you will have a mix of players from both ends of the spectrum, but that shouldn't prevent anyone from having fun. The non-competitives will tend to play amongst themselves, while the ones that are moving toward the other end will continue to accept the challenge of the more advanced players.
The thing you often have to push past in the younger crowd is the "my favorite Pokemon" phase. There is at least one decklist at every tournament where the Pokemon section overflows onto the back of the sheet, with plenty of ones in the Quantity column. Then they find themselves bewildered when they get trounced by better players. Some players will stay bewildered. Others will start to seek advice. That's when you want to introduce the concepts of synergy that make up a consistent deck.
- Try to drive home the fact that having more of the best Pokemon is substantially better than having one of everything. If they're advanced enough, dive into probability and basic statistics.
- Expound on the usefulness of Trainers. Show a Roseanne or a Bebe and demonstrate why it's good. If they're advanced enough for probability, then explain how 4 Pokemon plus 4 Bebe essentially gives you 8 chances to draw a particular card.
- Show why good decks don't need 30 Energy because they have other synergistic methods of getting to the Energy. Usually when I show the versatility of a card like Roseanne to a kid, a light goes on.
Most importantly, help them build! Don't come at it in a draconian way -- "you need to change X and Y and Z" -- but offer advice. If you've been beating them regularly, they will often be more open to it, as the play history becomes evidence that you know what you're talking about. Take the deck, sort it out on the table, look for areas of improvement, and start explaining changes that would help and why. Don't necessarily rebuild the deck for them, but try to get the concepts into their head. Sometimes you find that their "my favorite Pokemon" deck is not bad once you focus it on two or three key Pokemon lines.
Which reminds me, a good way to tell how advanced a player has become is just by pointing out a Pokemon that doesn't do much for the deck (there's always at least one), and asking the player, "What is this card doing for you?" If the answer is simply "I like him", then they're probably not ready for serious competition just yet. Kids that are actually thinking competitively (or at least starting to) will give you some strategic justification for the card, even if it's flawed. That's a good platform to start working from.
For the kids that I know are serious, I like to play some games with their hand face up on the table (particularly if I've just helped them build a deck and we're moving on to testing). This is where you can really get into advanced tactical discussion -- pointing out potential good moves, anticipating potential moves by the opponent, and disrupting the opponent's strategy. Like poker, there is a certain facet in Pokemon where it becomes less about playing your own game and more about countering your opponent's game. That thought process can start with something as simple as attaching Energy:
Me: "What do you think I'm going to do next turn?"
Him: "Attack and knock out [X]"
Me: "So maybe you shouldn't attach that Energy to [X] then?"
Over time they'll start watching themselves to avoid mistakes like this, and eventually they'll turn it around and start watching their opponents for similar mistakes and figuring out how to capitalize.
As with anything, it takes a lot of time and practice -- and most importantly, help. Make sure you're willing to invest if you want them to succeed.