A couple of articles (Handling the Underperformer on Your Team, & The Dirty Secret of Effective Sales Coaching) have good pointers on how to improve team efficiency:

One of the challenges that today’s busy managers struggle with is how to divvy up their precious people management time. Not everyone is a star performer so you should focus your limited bandwidth on the people who are doing the most for the organization, right? Unfortunately, high performers usually demand little time. They are self-sufficient, self-motivated, and often produce great work regardless of how much interaction they have with their manager.

Nor should the worst performers need a lot of attention.  In fact, focusing on the tails is the wrong thing to do:

Left to their own devices, sales managers often skew their coaching efforts dramatically toward the ‘tails’ — the very best and the very worst reps on their team. They engage with poor reps because they feel they must in order to meet territory goals, and they work with their best reps because, well, it’s fun. Few managers can resist the lure of reliving their glory days by passing along their wisdom to the one or two reps who remind them most of their younger selves. To combat managers’ tendency to coach just laggards and leaders, companies implement elaborate systems to allocate coaching equally across the sales force. They imagine that ‘all boats will rise’ as a result.

The recommendation is to focus on the middle 60% (Not the top or bottom 20%)

The real payoff from good coaching lies among the middle 60% — your core performers. For this group, the best-quality coaching can improve performance up to 19%.* In fact, even moderate improvement in coaching quality — simply from below to above average — can mean a six to eight percent increase in performance across 50% of your sales force. Often as not, that makes the difference between hitting or missing goals.

And how do you provide constructive feedback to improve performance: Provide detailed and constructive feedback.  Here are the four steps (pretty intuitive)

1. Diagnose the issue. Understand what can improve performance: lack of motivation, skill deficiency, misalignment with goals, personal conflicts, or home/ family issues. 
2. Share what you are seeing. Discuss with the employee, Be specific and use examples. Ask for the employee’s perspective
3. Specify necessary changes. Decide what needs to change and how he should go about changing. It’s critical to set up processes by which the performance can be improved. Set clear goals and timelines. If you’re not sure how to support him, ask for help from an HR partner or an external coach.
4. Evaluate and take action. If the performance improves, congratulations.  If not, take action.  

Here are some important factors to consider if the employee is a perennial under-performer and you want to take action:

Reflect on the person’s value to the organization. He may be invaluable in one arena but underperforming in another. Can you change his job description so that it better plays to his strengths? Or can you find another position in the organization that’s better suited for his skills? If the answer is no, you may need to terminate. Of course, making a firing decision shouldn’t be taken lightly. Whatever you decide, don’t leave it up to them as to whether they leave. That’s a surefire way to create deadweight and hurt the morale of your team.

Here are related posts that you might find relevant: Mentoring, Performance Reviews, & Team Performance.