1. Think long-term

Buffett has preached the merits of thinking long-term throughout his career. He once said, "Only buy something that you'd be perfectly happy to hold if the market shut down for 10 years." He also has said, "If you aren't willing to own a stock for 10 years, don't even think about owning it for 10 minutes."

This doesn't mean that Buffett always owns stocks for years; he doesn't. But he always has a long-term mindset.

2. Understand the underlying business before buying a stock

The legendary investor has also consistently emphasized the importance of understanding the underlying business before buying its stock. In his most recent letter to Berkshire Hathaway shareholders, speaking of himself and longtime business partner Charlie Munger, Buffett wrote, "Charlie and I are not stock-pickers; we are business-pickers."

3. Look for durable competitive advantages

One specific thing that Buffett encourages when evaluating a business is to look for competitive advantages that are durable. Here's how he once put it:

4. Invest in wonderful companies at a fair price

In his early days, Buffett was a dyed-in-the-wool value investor. He still focuses heavily on valuation. However, he believes that the combination of the quality of the business plus valuation is even more important. As he once said, "It's far better to buy a wonderful company at a fair price than a fair company at a wonderful price."

5. Know what you don't know

Few investors do the research on a company that Buffett does before he puts any money on the line. But even the Oracle of Omaha recognizes that he can't know everything, and he has stressed the importance this: "What counts for most people in investing is not how much they know, but rather how realistically they define what they don't know."

6. When to be fearful and when to be greedy

One of Buffett's most famous quotes is, "Be fearful when others are greedy and greedy only when others are fearful." He doesn't believe in blindly following the crowd with any investment. The best opportunities can come when most other investors are too afraid to jump in.

7. Take advantage of opportunities

Buffett is cautious by nature, which has worked to his advantage. But he definitely supports taking advantage of opportunities, or as he puts it: "Opportunities come infrequently. When it rains gold, put out the bucket, not the thimble."

8. It's OK to sit on the sidelines

Although Buffett prefers to be heavily invested in stocks, he thinks it's OK to sit on the sidelines when there aren't attractive opportunities. In particular, he won't buy many stocks when valuations are too high. He used a great analogy in the past to describe the appropriate mindset: "An investor should act as though he had a lifetime decision card with just 20 punches on it."

9. Don't fret when the market falls

During his 93 years, Buffett has lived through plenty of stock market declines. He has provided excellent advice on how to view market pullbacks:

10. Recognize when you've made a mistake

Buffett emphasizes buying and holding stocks over the long term. However, he doesn't have any problems selling stocks, especially when he's made a mistake in evaluating its business prospects. In his words, "The most important thing to do if you find yourself in a hole is to stop digging."

11. Invest in yourself

Not all of Buffett's investing advice has been about investing. One of his most important nuggets of wisdom is: "The most important investment you can make is in yourself."

12. Buffett's best financial advice of all

I couldn't end this list without including what's arguably Buffett's best financial advice of all: "Rule No. 1: Never lose money. Rule No. 2: Don't forget rule No. 1." Consistently following this advice, though, is certainly easier said than done.