The first is finding the right need. Remember, the effectiveness of training must be measured not by whether it was delivered, but by the effect it had on people's performance. By that criterion, Digital Game-Based Learning is, in many instances, clearly the best and most effective way to go. Of course one of training's dirtiest little secrets is that this is what most of training is about - if people wanted to learn the material, they would, on their own or from their peers. Once managers actually understand and accept that their people don't want to learn this stuff but have to, convincing them becomes easier.
Managers are also often able to relate to the generational changes through their own children.
You can ask them if their kids are very different from them, which they all admit, and then point out to them that most of their trainees are more similar to their kids than to them. The fact that managers are getting younger, especially in some industries, helps as well. Finally, the whole point of my book is to help trainers make this case. There is also an entire chapter on Evaluating Effectiveness. Interestingly, the stakeholders most difficult to convince are, typically, the trainers and educators.