1) It takes more than brains.
Doctors are notoriously poor investors. The same could probably be said for many highly intelligent people. The problem here is overconfidence. An enormous amount of market behavior is due to unforeseeable events. In the short term most of its action is random. Believing that you know more than the next guy may lead you to place too much money in one trade or take on too much risk. Nevertheless a reasonable level of intelligence is required, and basic math skills are essential. If you don't understand basic accounting principles, you'll find it impossible to size up a company's prospects.
2) Insecurity and a tendency to follow the herd are lethal.
Ask yourself a question, and try to give an honest answer. Do you tend to look to authority for most of the solutions to your problems, even when it's not clear the authority knows any more than you do? Do you do whatever your friends are doing and follow the fads? If so, managing your own portfolio is not for you.
To be successful, you need to study your stocks until you're convinced you know as much or more than the pros about those particular issues. You also need to have the courage of your convictions even when the market doesn't immediately confirm your judgment. If you sell every stock that goes down and replace it with one the pros recommend, you'll probably do worse than if you'd been patient-assuming of course that you practiced due diligence in the first place.
3) It helps to be a little girly.
Several recent studies show that women tend to outperform men in investing. Of course, this only applies to women who play the game. Women who succumb to stereotypes about females being financially incompetent or who avoid the stock market altogether undoubtedly do worse. Women who participate in the market tend to do careful research and take calculated risks, whereas men sometimes indulge in testosterone-fueled frenzies of reckless behavior that damage their bottom line. They also tend to trade too much, which eats up profits with commissions and taxes. If, as a recent book title suggests, Warren Buffett Invests Like a Girl, maybe you should too.
4) It takes time.
If you don't have a few hours a week to monitor your stocks and scout new prospects, you're better off investing in broad-based index funds.
5) It takes guts.
Can you stay calm when everyone else is panicking? Can you boldly buy stocks when the market is plunging? If not, you'll have a hard time beating the market.
6) You have to like money, but not too much.
If you don't enjoy having and spending money, you won't be interested enough to do the necessary research, On the other hand, if you place so much importance on money that the possibility of losing any at all makes you break out in hives, you won't be able to take the risks you need to succeed.
7) It takes patience.
In spite of instant trading programs and mountains of available data, stocks get stuck in downtrends that don't reflect their true value and in uptrends that grossly inflate it. If you know a stock is undervalued buy it, but be prepared to wait for your bet to pay off.
Some people can consistently beat the market, but they are in the minority. It takes an unusual combination of personality traits and a significant time commitment to be one of those lucky few.