Businesses have tried everything from integrating technology to creating new pitches in an effort to eke more value out of their sales departments. However, as the Sales Executive Council (SEC) has noted in past studies and research, there is no other productivity investment that approaches the benefits of sales coaching when it comes to bolstering activity.

However, sales coaching doesn't impact everyone the same. In fact, it can even be a wasted investment in some scenarios. For example, additional research from the SEC suggests that trying to coach the worst performers provides little return, as these individuals are likely poor fits for the job in the first place. Sales is a cutthroat industry, and it's best to let these underperformers realize for themselves that they aren't suited for the job.

At the other end of the spectrum are the top performers. These individuals are already exceptional at their jobs, and as a result, have less room to grow. While coaching can still help your best salespeople, it may not provide the greatest return on investment.

So what is the secret to effective sales coaching? Brent Adamson, senior director of the SEC, suggests it's targeting the right audience: the middle-of-the-pack sales agents.

"The real payoff from good coaching lies among the middle 60 percent – your core performers," he wrote for the Harvard Business Review. "For this group, the best-quality coaching can improve performance up to 19 percent. In fact, even moderate improvement in coaching quality … can mean a 6 to 8 percent increase in performance across 50 percent of your sales force. Often as not, that makes the difference between hitting or missing goals."

So much focus is placed on how sales leaders coach their employees – which training mediums they use, how to schedule coaching, whether to use a third-party service provider, etc. When it comes down to it, the question is as much "who" as it is "how." Sales departments need to hedge their bets with the people who will provide them with best returns on investment. For many businesses, that won't involve focusing on the bottom end of the spectrum, but rather the middle-of-the-road performers.