Every investor expects to make a killing in the stock market. They expect to buy low and sell high. Unfortunately, the opposite holds true.Investors tend to buy high, and sell too low, because they're inclined to buy when the markets are rising and sell when the markets are falling. Buying and selling behaviors make it difficult to beat the market. The problem is not in the market, but in the behavior of investors. Our emotions always prevent us from making rational decisions.Every investor tries to "beat the market." Beating the market means trying to earn an investment return greater than that of the Standard and Poors 500 Index, a popular benchmark of the United States stock market. Very few do.There's an old saying on Wall Street: Bulls make money, Bears make money, but Pigs get slaughtered. It warns the investor against excessive greed. Buying stocks on Wall Street can be a risky business. Stocks can go down as fast as they went up, so if you have a profit, you should always take it. Profits on paper look good, but cash in your hand feels a lot better. Always turn a profit into cash when you have one. Never let a profit turn into a loss. No one ever went broke taking profits. Profits can always free up cash for new trades, and it avoids the risk of letting profitable trades go bad.As humans, we all make mistakes; making mistakes in the market can be very expensive. There is no profit in being wrong. The side that makes the least mistakes is the most successful. The goal of every investor is to have small losses and large gains. You don't have to be a market genius to succeed in the market. All you need is a little common sense, reliable information, discipline, and patience. The overwhelming evidence is that stocks will remain the best investment for all those seeking steady long-term goals. Buy quality - not quantity.As Warren Buffett said: "Investing is simple - but it's not easy.