Here's an article from Kiplinger's where they promise to share four mantras of successful investing. Here's the first one:
Mantra 1: Think long term
If you're looking at retirement 30 or more years away, or another major goal that's a decade or more in the future, then you have plenty of time on your side. And a long time horizon will help smooth out the market's day-to-day and year-to-year gyrations.
The longer you have until your savings deadline, the more risk you can afford to take. If your goals are decades away, consider putting all your money in stocks or stock mutual funds. You'll experience some swings in the market, but if you hold fast to your strategy, it should pay off in the end.
Your 20s and 30s are when you build the foundation of your wealth, and if you invest too conservatively now, you may severely hobble your growth potential over the years. For example, a single investment of $5,000 earning 10% each year would be worth $226,300 in 40 years. If your money earned 6% a year over those 40 years, you'd end up with only $51,400.
Some good points here. I'm just into my 40s and am about 97% in stocks (through index mutual funds). I'm fairly aggressive and don't see changing this mix until my late 40s or early 50s.