Gallup has been tracking employee engagement over the past 17 years, and its research reveals that roughly 30 percent of U.S. workers are considered "engaged" in their jobs.

Translation: They are inspired by, involved in, enthusiastic about, and committed to their work. These are your star employees, who release discretionary effort because they're emotionally connected to the job.

Roughly 50 percent of employees, in case you're curious--and you should be, because they are working for you right now--show up to work, watch the clock waiting for lunch and 5:00 p.m. to roll around, kill time, and do the minimum required, with rare extra effort.

You don't want to hear what the remaining workers do. OK, I'll tell you, but don't shoot the messenger.

Gallup says roughly 17 to 20 percent are "actively disengaged." These employees are walking zombies (my term); they're either checked-out or just going through the motions. And they may be working for you right now, infecting the staff's morale.

So how do you keep your star employees, the ones who make up that rockin' 30 percent club, from being infected by the other 70 percent, and quite possibly leaving you for your competitor?

The research behind Fortune's "100 Best Companies to Work For" (which documents strategies for having very happy employees and bottom-line impact) finds that the work environment at these companies focuses on growing a high-trust culture and developing people. Michael C. Bush, CEO of Great Place to Work, recently said:

The new, largely uncharted business territory is about developing every ounce of human potential, so that organizations can reach their full potential. All companies--including the "Best Workplaces"--face the challenge of creating an outstanding culture for everyone, no matter who they are or what they do for the organization.

The Most Effect Way to Keep Star Employees From Leaving? Pick One.

What you'll find in these "Best Workplaces" is a lot of freedom to make decisions, great communication across the organization, and a culture in which you never stop growing as human beings. You can't go wrong with any of these, but if I were a betting man, I'd wager on No. 1 on the list. This will give you clear competitive advantage.

1. Make your employees feel like business partners. Employees thrive in intrapreneurial settings, which make them feel like they're invested in the company. This means giving them freedom in and ownership of their work. When you do, they're likely to perform at a higher level.

2. Be transparent. Have open communication channels in your boss-employee relationships. Let your staff get familiar with what's going on in the business and how their work impacts their customers. Give them a chance to have input and suggest ideas.

3. Expose them to new responsibilities. Don't be afraid to stretch them with work assignments that will expand their knowledge and sharpen their skills.

4. Treat them with respect and dignity. Get to know them on a personal level, even outside of work. Show them that you don't just view them as worker bees but as human beings.

5. Get their feedback. Don't just put a "program" or product together and leave the scene. Constantly ask your employees for feedback, what's working and what's not. If you want to maintain a culture of happiness, you have to keep your finger on the pulse.

6. Let your star employees mentor a peer or colleague. One study revealed that workers who help others (as little as 10 to 30 minutes per day) feel more capable, confident, and useful. They feel they've accomplished something, and therefore that they can accomplish more in the future.

7. Create a learning environment. Similar to No. 3 on the list, allow your star employees to explore opportunities inside your company to learn something new, such as joining a cross-functional project, picking up another skill, or leading or participating in a "lunch and learn."

8. Never stop attaching meaning and purpose to their work. Set short- and long-term goals that your star employees will be passionate about pursuing. Adopting Google's famous 80/20 policy (the effectiveness of which, however, has been hotly debated), let your star employees dabble in their own innovative pet projects that align with a company strategy or vision, for 20 percent of the time. Scale it back to 10 or 5 percent, if needed.

9. Create a culture of recognition and praise. Let your tribe nominate one another for special recognition for innovation, creativity, hard work, and going above and beyond. Then share the success story with the rest of the company. In one large Gallup study, employees who received praise and recognition regularly increased their individual productivity, received higher loyalty and satisfaction scores from customers, and were more likely to stay with their organization. How regularly are we talking? Praise should be given once per week.

10. Give them a voice. A great example of giving your best people a voice is found at Acuity, the property/casualty insurer. Its employees participate in annual strategic planning, deciding on the charity organizations Acuity will donate to. Acuity's freedom-centered work environment, in which employees have a voice in really important decisions, has led to an incredible 2 percent voluntary turnover rate.