Most HR departments already know that today's younger employees--the Millennials--tend to job hop. The Bureau of Labor Statistics backed this up in its September 2014 report, finding that the median tenure of salary workers is just over 4.5 years--about the time a typical student spends earning an undergraduate degree.

But while we could go on and on about why this is happening (and there are numerous reasons), I like to look at this as a recruiting and leadership opportunity. Instead of worrying how to extend the length of your overall employee tenure, just assume you've got four years with each one. Then ask yourself: How should this "four year sweet spot" influence the way my company recruits? And how can I lead employees most effectively during those four years (and beyond), so I can improve the company's return on its hiring investment?

These five tips should get you started.

1. Always be recruiting your new "freshman class."

If today's typical employee spends about as much time at one job as he or she does in college, then--if you'll indulge my metaphor--you must continuously be bringing in new "freshmen" to replace your outgoing "graduates." Sure, not all of your "students" will leave. Some might stay on for a fifth year to earn a Masters. Even fewer will stay seven years to earn a PhD. But the statistics conclusively show that a lot of folks will leave you around their four-year mark, so you better be filling your admissions pipeline.

2. Use interviews to predict the future.

Obviously, you need to discuss a candidate's employment history during an interview. But remember that you're not just sussing out skills--you're trying to determine if this is a person who routinely jumps between companies every two years without making meaningful contributions.

Habitual short-timers who don't contribute should be avoided. You simply won't get any return on your investment in hiring and training them. Not only does it cost money to recruit and replace people, but if you put a person in a job and they walk away too soon, you're losing value in productivity. And don't be fooled by people who spend a decade with an employer if they have nothing to show for it. Longevity doesn't automatically provide return on investment; you need productivity to complete that equation.

Be sure to ask candidates about their career goals and see if they match what you're looking for. Are they looking for excitement? Advancement? Stability? Money? Purpose? Do their objectives and their resumes suggest a likely commitment for a productive four years? If not, steer clear.

3. Accelerate your training and onboarding.

The faster you get a new hire up to speed, the faster they can become productive--and the faster they can begin delivering ROI. Clearly, if it takes your company a full year to train and onboard new employees, you're using up 25% of that person's potential tenure on introductory processes. Figure out how to speed that up without compromising the quality of your onboarding. Often, that might involve making an investment in new software tools or mentoring programs--but if you can facilitate the learning curve and put new hires in a position to get up and running faster, you'll reap returns that much sooner.

4. Know who's moving on--and who's not--so you can focus leadership appropriately.

After employees have been trained and assimilated into your work culture, they should be hitting their stride. For the next two to three years, you should be seeing productivity, growth, and returned value. This is the time to watch people. What are they thinking? Are they enjoying their work? Displaying ambition? Getting fidgety? It's your job to spot the folks who will, for whatever reason, opt to graduate and leave--as well as those stars who might stay a few more years to pursue an "advanced degree" at your company. Whenever you can spot those star employees, start further investing in them--not only with your time, but with new projects and career advancement. You know they're open to staying, so make it worth their while.

5. Cultivate good relationships with your "alumni."

When good employees "graduate," it's typically because they want to grow their careers and build experience. Don't begrudge them that choice. Instead, make sure your "alumni" leave on a good note. Use a database to maintain strong relationships and communicate with them regularly. And be eager to help them in their new adventures--because every time you serve as a positive reference, you're increasing the chances that they will return the favor by referring their friends and connections. And who knows? Perhaps one day they'll come back to you with new skills, stronger than before.

We can't keep our best employees forever, especially in today's competitive job market. But, with the right planning, we can extract the best value from those employees--even after they've left.