In our organization, we have training departments whose job it is to transfer knowledge to new team members. They don't limit themselves to just one form of communication medium.
We employ a mixture of one-on-one training or group training, which can be conducted either on site or through video calling tools such as Google Hangouts or Skype.
In addition, we've created hosted video tutorials, which we follow up with quizzes to help the employee test their knowledge. The quizzes give the employee the opportunity to go back into the video and review anything he or she may have missed, and they also give our trainers feedback in regards to who may need more individual coaching or mentoring. The training videos are put together using open source desktop recording software such as CamStudio.
Finally, we host the videos in a Google Site or MediaWiki, depending on departmental choice. The former is obviously much easier to setup, as it only requires a Google Account. Organizations using Google Apps for Your Domain can easily share Google products with members of their organization using Google accounts but with administration roles delegated within the organization.
The videos hosted on the Google Site are generally supplemented with documentation, step-by-step instructions, and much more detailed information that one might find useful.
Our goal is to not only provide our employees with a way to obtain knowledge, but also to ensure that the knowledge is transferred using the method that will maximize knowledge acquisition in a manner that is fun and interesting and that also acknowledges the fact that not everyone learns the same way.
Developers typically go through different classes with different senior developers where they work on smaller projects until the senior developers feel they're ready to take on more responsibility and work more independently.
The key though is to create a work environment that reduces the amount of turnover. Without a great work environment, it will be difficult to attract and keep good people. My suggestion is to also spend some time trying to determine why people are leaving. Patching the hole in the bottom of the boat may yield the best results for your particular situation.