Warren Buffett is offering some basic instinct advice about investing.

In his annual letter to Berkshire Hathaway shareholders to be released Saturday, he writes about how nonprofessional investors shouldn't get distracted by constantly changing stock prices and instead buy stocks of companies that will be profitable over the long run.

After warning against entering the market "at a time of extreme exuberance and then (becoming) disillusioned when paper losses occur," Buffett quotes the late Wall Street money manager Barton Biggs:

"'A bull market is like sex. It feels best just before it ends.'"

In the excerpt from his upcoming letter, published by Fortune, he goes on at much greater length about his "fundamentals of investing."

(Read more: Berkshire Hathaway's 15 biggest stock holdings)

Berkshire plans to release the letter at about 8 a.m. ET Saturday morning.

—By CNBC's Alex Crippen. Follow him on Twitter @alexcrippen.