Short selling is a kind of reverse stock buying. Normally, you purchase a stock with the thought of it going up in price. But in short selling, you actually sell a stock before you buy it with the thought that it will go down in price.
As with normal stock investing, short selling requires keeping a vigilant watch on the general market indices. Knowing what the market averages are doing will give you clues as to whether stocks may be topping out, or if the markets condition is conducive to short selling. If you're wrong about the market's direction, be sure to employ a loss-cutting strategy similar to regular stock investing.