The long-term view
A moment ago I realized why I don't see eye-to-eye with the folks who say that if you buy and hold for 40 years, you're going to do alright. It's not that there's any evidence that they're wrong. It's just that the evidence that they're right is based on a mere 100-150 years of data.
That is to say, just because the U.S. economy has always recovered from what ails it within 20 years or so, that's not proof that it always will. In fact, the real long-term view (4000+ years of recorded human history) teaches us that short-term recoveries in a civilization's economy are always followed by total collapse, or protracted insignificance at best. Egypt was practically wiped out and remains insignificant today. Rome was wiped out, and has ceased to be an empire for more than 1500 years. The Aztecs - wiped out. The Mohammedan Empire - faded and receded into insignifance (sorry, DWI). Even China has taken its sweet time reclaiming its old strength and global significance. In the 1800's and 1900's China was the economic runt of the Asian litter.
You might say that this proves nothing. Well, then: what does it prove that an investment in the U.S. stock market at any time in the last 100 years was handsomely rewarded if held for 25 years or more? Maybe the last 100 years was the U.S.'s economic heyday. Maybe we've got another 100 years of prosperity after this economic downturn passes. You don't know.
Nothing of this world is eternal. That goes for the U.S. stock market at least as emphatcally as for anything else.
Keep paying attention to the fundamentals of what you're investing in. I like how Peter Schiff puts it: there's a bull market somewhere. We may not have another one here until our children's children's children start to die of old age. But there will always be opportunity somewhere for a shrewd, alert investor.