One of the reasons I returned to the airline business in 2007 was to help revive Delta’s historic employee-focused culture. Nearly 70 years ago, the airline’s founder, C.E. Woolman, put together a simple list of values for new employees that focused on integrity, honesty, support and compassion for coworkers while always putting oneself in the customer’s shoes. Those values were the bedrock of the culture Delta (DAL) refined over the years — one that helped the airline establish a reputation for its hospitality and service.

That culture was strained during the dark years after 9/11. But even at the most difficult times, Delta’s employees kept the enterprise going. The culture, more than anything else, saved the company. Delta’s people took care of each other and their customers, even when they didn’t always have the support of their management.

It sounds easy. Many companies talk about putting culture first but eventually find that it’s hard to achieve. Here are some of the lessons we learned when it comes to building and maintaining your culture:

Know who you are
Don’t try to copy the culture of another enterprise just because it’s successful or has been in the news. Every organization has shared values that are unique to its people and its mission, and it’s worth taking the time to recognize and understand them. Imposing something from the outside on your people just won’t work, and will probably make things worse, not better.

Employee buy in is essential
This means more than just handing out booklets to new hires. Your people must feel in their gut that their daily work is crucial to the company, and that they will benefit from success.

At Delta, that means offering strong financial incentives to employees. We pay every employee a bonus if the company achieves operational and performance goals.

See also: You get the company culture you are willing to accept

Management must be on board
Sometimes leaders talk a good game about values, but their actions in the boardroom don’t match up. At Delta, we have a policy in executive meetings that anyone who suggests something that violates our rules has to throw some money into a pot at the center of the table — it’s basically a “swear jar” for values. If your management team considers themselves above the rules, your employees will notice and your culture will suffer.

See also: Here’s How Much Lower Manhattan Has Changed Since the 9/11 Attacks (2016, Fortune Editors and Reuters)

Keep tending the garden
If you’re successful, it can be tempting to assume the culture is strong and will always be there. But things have a way of slipping once you’ve achieved your goals. That’s why we raise the bar on our plans every year and periodically revisit our rules to make sure they are still relevant. Management meets regularly with employees across the globe to get their feedback and talk about important issues.

Hopefully, if you always remember to put your culture at the center of what you do, it will serve your employees, customers and investors very well for many years.

Read all answers to the Fortune 500 Insider question: How do you build a company’s culture?

What 23 years at the same company taught this CEO by Alan Colberg, president and CEO of Assurant.

What a tornado taught this team about workplace culture by Deb Aldredge, chief administrative officer of Farmers Insurance.

So you messed up at work. Here’s what you do next by Val DiFebo, CEO of Deutsch New York.

How Wells Fargo’s CEO built the team at the world’s most valuable bank by John Stumpf, chairman, president and CEO of Wells Fargo & Company.