I used to think that company culture happened naturally.  After starting and building five companies, I've learned that great culture doesn't just happen—you need to make it happen.  In general, a company's most expensive asset is its people.  So it surprises me that so many companies fail to develop a culture or "people plan" to invest in and grow that asset.

When I started my most recent venture, the Rubicon Project, an online marketplace for buying and selling ads, the first thing I did was create a blueprint for our culture.  I talked with the founding team about the kind of organization we wanted to build and the values that we'd instill to guide our employees.  

We didn't start with a business plan, product roadmap, or marketing budget.  Why?  Well, what I have learned is that as you're growing a business, everything around you is constantly changing.  The market, the product, competitive landscape, and economy all change.   Your business plan and product are far easier to evolve than your people.  I firmly believe that the difference between a good company and a great one is the strength, passion, and loyalty of its people. 

Here's how to design your "people plan":

1. Write a mission statement. People are driven by causes more than anything else.

2. Define company values. These are the guiding principles for how you expect your team to behave internally and externally. Put it on the wall or on your mouse pads.

3. Build a culture roadmap. Take the same approach as business or product planning. What tangible things will you put in place to promote and grow your culture? Think communication tools, team building exercises, team bonding events, for instance.

4. Measure and adapt. Take a survey every quarter and ask for feedback and ideas. SurveyMonkey is a great online tool for this.

5. Make culture a priority. Sounds simple, but I haven't met an employee at a growing company who doesn't have more work to do than time to do it.  Remind everyone to live by your cultural values, and prioritize them in communication, hiring, and everyday work.  Give employees ownership of culture, and ask them for their help.

6. Form an interviewing committee. Create a cross-departmental team that represents your culture well. Set an intangible standard for hiring (communication or personality, for example) and make it a policy that a member of the culture committee must interview every prospective employee. Larger companies can scale this by limiting it to management positions.

7. Establish a development committee. Select a diverse group of people from every department (and office) in the company and challenge them to come up—every week—with a new culture idea that can also be implemented in a week.  This creates momentum and constant improvement. Change one-third of the members each quarter to keep it fresh.

8. Communicate, communicate, communicate. Constantly remind people at every team meeting, in every operating plan, on email and in person that culture is a priority to you and that they are all part of it.

Companies like Google, Zappos, and Southwest Airlines are famous for their cultures and attribute much of their success to it. Great culture doesn't have to be about hiring chefs or riding around the office on scooters. Here are some more ideas to help get you started: randomly assign lunch tables to get different people to interact with each other, organize a community service event, put your cultural values on your website, ask everyone about what they like about working at your company, record it on video, and then show it to everyone.

Your most important and most expensive asset walks out the door every day. Give your people an authentic reason to come back.