How to prepare academically-minded students and postdocs for the eventuality of not finding secure academic employment? Graduate students and postdocs are predominantly trained to be -- and driven by the prospect of becoming -- full-time academics. For a majority of them, there comes a point of reckoning when they realize they may not be among the select few that "make it" to a secure full-time academic job. This realization is commonly accompanied by the feeling of having been exploited by the academic system, a feeling that becomes more and more exacerbated the longer one remains in graduate studies or post-graduate contingent positions before moving on to a different field. This is both well-studied and quantified in higher-ed literature and virtually omnipresent as anecdotal accounts on higher-ed media.

As a soon-to-be assistant professor, how can I prepare academically-minded students and postdocs for the eventuality of not finding secure academic employment? Are there well-documented mentoring methods on this subject?

I am more interested in what I can do individually, rather than how to force institutional change collectively, as I have found it easier to find resources on the latter topic.
A few of your students may just study your subject for the pure love of it. More of them, I suspect, will settle for nothing but an academic career. You may not be able to convince them to aim otherwise. 

But, you can still, with some effort, show them all what the alternatives are. In particular you can do two things, one simple and the other harder. 

The first is to develop a list of sources in various industries that utilize the sorts of things you teach. Invite someone to visit your class about one day a month and to talk about the sorts of applications that are done in their industry. Even better if they can talk about their own personal involvement. It will take a while to get started in this, but a contact might be willing to return annually. You can expand this, possibly, if there are several sections of your course, even those taught by others, and have a late afternoon or evening seminar with such a guest. 

The second thing is to develop, over time, an internship program with local industries, where students visit the companies and maybe perform some tasks there. But just visiting the companies to see what life might be like in industry might be enough. Many universities have internships and some depend on them as a fundamental part of the education, so you might be able to get some help in this. 

One of the problems of the future academic mindset is lack of knowledge of the alternatives. You can give them at least a hint of that.