Warren Buffett has amassed a following of millions who've learned, like Buffett, that success is achieved by making smart investment decisions. But those decisions may have nothing to do with stocks and bonds.  

In a 2019 interview with Yahoo Finance editor-in-chief Andy Serwer, Buffett said: "By far the best investment you can make is in yourself."

This may be the most valuable of all lessons from the Oracle of Omaha. To make the most of your investment means never stop acquiring knowledge -- the kind of knowledge that betters yourself as a whole person, not just as an investor.

Here are three ways to do it:

Invest in improving your communication skills

In a video posted on LinkedIn, Buffett said,

If you can't communicate, it's like winking at a girl in the dark--nothing happens. You can have all the brainpower in the world, but you have to be able to transmit it. And the transmission is communication.

He added that investing in developing your communication skills -- both in writing and in-person -- "can increase your value by at least 50 percent."

Invest in improving your leadership skills

Buffett has no shortage of advice about successful leaders. He has been telling us about the importance of one non-negotiable trait you should find in every leader: integrity. The Oracle of Omaha once said:

We look for three things when we hire people. We look for intelligence, we look for initiative or energy, and we look for integrity. And if they don't have the latter, the first two will kill you, because if you're going to get someone without integrity, you want them lazy and dumb.

Tony Simons in The Integrity Dividend argues for integrity as the predominant characteristic that "touches every aspect of your business." When practiced in its best form, says Simons, integrity enhances the value of the business and drives profitability.  

Invest in improving your business relationships

One of the best strategies for success is to surround yourself with the right people. Buffett spoke with some college students a few years back and said, "You will move in the direction of the people that you associate with. So it's important to associate with people that are better than yourself."

As the famous saying goes, we are the average of the five people we spend the most time with. Make sure to associate with those further along the path who can potentially help you learn new things, grow, and advance your career. 

Buffett's lifelong pursuit of learning and investing in himself, which he shares with his longtime Berkshire Hathaway partner and colleague Charlie Munger, is the secret sauce of his success.