It’s almost become a cliché: Past performance does not guarantee future results. But it’s a lesson too many investors ignore.

Just because a certain mutual fund has performed exceptionally well during the economic slump, doesn’t mean that it will enjoy the same returns once the economy recovers. Just because a certain stock has been rising steadily, doesn’t meant that next week it won’t crash.

The unfortunate truth is this: No one can predict what any mutual fund or stock will do tomorrow, next week, next month or next year. The performances of these vehicles rest on too many factors. Stocks, after all, rise or fall often on rumors and innuendos. A mutual fund’s value may dip or soar based on events that happen in foreign countries thousands of miles away.

This isn’t to say, though, that investing is purely a game of chance. It’s not. Wise investors know how to hedge their bets, and protect themselves financially as much as possible. They know how to spread the investment dollars over a diverse array of stocks, bonds and other investment vehicles so that if one investment struggles, the success of others will cover it.

Too many other investors, though, invest their money on hunches. They may see that a certain stock has been a strong performer for a year and move vast sums of their investment dollars from other vehicles into that steady performer. They’re then shocked when the stock eventually begins to struggle.

The best way to approach investing is to treat it like a job: This means you have to stay abreast of financial news by surfing the Web and visiting the most respected financial advice sites. It means working with a trusted financial advisor who can help guide you through the often challenging world of investing.

And it means, most of all, recognizing that no investment, ever, is a sure thing. Investment always involves a certain amount of risk. Even the hottest funds and stocks can lose value. Even the steadiest of performers can enter a slump. And, despite what some people might claim, there’s no real way to predict when a hot performer will suddenly turn cold.

This brings us to the last big rule of investing: Only invest what you can afford to lose. That does sound a bit like gambling: You don’t bet more than you can lose. The big difference, though, is that investing requires serious study and planning.