Warren Buffett has been dropping investing wisdom on the masses for over 50 years. Each year he publishes a letter to Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B) shareholders, he speaks to the media, and he puts his money where his mouth is by investing cold, hard cash. 

Today, we'll dive into Buffett's philosophies of the past 15 years to distill the top lessons that can be applied to investing right now. 

But first, a little context
It should come as no surprise that Buffett's tactics have changed a bit here and there over the decades.

During that time, though, he has stuck to the same core value-investing principles. He still points first and foremost to the fundamentals learned as a student of Ben Graham: focus on intrinsic value, have a margin of safety, understand the business's fundamentals, and so on.

Times might change and his ideas might evolve, but Buffett remains grounded in the core value-investing tenets that launched his career more than five decades ago.

Lesson 1: Don't be afraid to change your opinion as circumstances warrant
Historically, Berkshire has never repurchased shares. Instead, Buffett used that capital to invest in portfolio companies or expand equity holdings. Then the financial crisis hit, and Berkshire's market value fell to a point that justified reevaluation of the "no buyback" policy. In his 2013 letter, Buffett explained the previous year's stock repurchase:

As I've long told you, Berkshire's intrinsic value far exceeds its book value. Moreover, the difference has widened considerably in recent years. That's why our 2012 decision to authorize the repurchase of shares at 120% of book value made sense. Purchases at that level benefit continuing shareholders because per-share intrinsic value exceeds that percentage of book value by a meaningful amount. We did not purchase shares during 2013, however, because the stock price did not descend to the 120% level. If it does, we will be aggressive.

Circumstances changed, and so did Buffett. The lesson? We should all constantly analyze, tweak, and revise our investing tactics.

Lesson 2: Partner with and learn from the best and brightest people you can find
It's fairly common to hear stories of famous celebrities or athletes, overwhelmed with the challenges of overnight riches, coming to Omaha in search of advice. It's far less common to hear stories of Buffett reaching out to others to learn something new himself.

But Buffett's career is littered with instances of partnering with and learning from the smartest and most successful individuals in investing and business. 

In his early years, Buffett's teacher and mentor was Ben Graham. Later, Charlie Munger taught him the wisdom of paying a premium to own the best of the best. More recently, Buffett has expanded his repertoire to include several partnerships with the ruthlessly efficient private equity group 3G Capital.

Some might question the 3G partnership specifically, as Buffett has long denounced private equity. The reality, though, speaks to the first lesson above.

Buffett kept an open mind, learned how 3G was different than its peers, and then invested accordingly. He highlighted 3G's excellent performance in an interview on CNBC earlier this year.

There is no finish line in either Berkshire's investments or 3G's investments. So, we've got a wonderful partner. I knew they were wonderful going into the Heinz deal, and in every respect, both in terms of ability, but just in terms of integrity, every aspect of it, 3G has been a perfect partner. ...

3G -- they're not buying things to sell. And the other private equity firms that have term limits and all of that, they have a time when they are going to sell something. And they buy companies with the idea of either IPO'ing them, or selling them to competitors, or selling them to another private equity firm. That is not 3G's strategy. I don't think it's even proper to call them a private equity firm exactly because they're buying to keep just like we're buying to keep. 

This example is fantastic because it highlights both lesson one and also lesson two: find the smartest, most successful people and do everything you can to work with them and learn from what they do.

Lesson 3: Watch your back (aka manage your risks!)
The stock market is in the sixth year of a bull run. For some investors, that lengthy climb from the lower left to the upper right has erased the painful memories of 2008 and 2009. But to forget completely is a huge problem.

The U.S. and global economy is neither linear nor static. Throughout history the economic cycle has been characterized by expansions followed by contractions. Two steps forward, one step back.

With decades of experience to lean on, Buffett understands this reality and the psychology that drives it. In his 2000 letter to shareholders, he explained: "The line separating investment and speculation, which is never bright and clear, becomes blurred still further when mot market participants have recently enjoyed triumphs. Nothing sedates rationality like large doses of effortless money."

In other words, the good times won't last forever. Avoid speculation by thinking of your investments as businesses instead of stocks. Anticipate and mitigate risk. And do your absolute best to keep a level head when the market does turn.

If you can do that, those bear markets become your buying opportunities. It has worked for Buffett, and I bet it will work for you.

Jay Jenkins has no position in any stocks mentioned. The Motley Fool recommends Berkshire Hathaway. The Motley Fool owns shares of Berkshire Hathaway. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.