Successful growth investing requires either a rock-solid refusal to pay too much for growth or a quick finger on the trigger so that you trade out of a stock at the first sign of a slowdown in growth. Instead, they study the movement of a stock's price over time in an attempt to find patterns in past prices that will predict future prices. Relative strength is one common measure used to track how a stock is performing in comparison to the rest of the market. More complex strategies in this family rely on technical analysis. Using charts of past stock prices and such additional measures as the daily trading volume, technical analysis attempts to find patterns of price movement that will predict where a stock's price is headed.
Technical analysis is too complicated an art to cover here.
You can learn more from one of the books listed below. Price-momentum strategies don't work particularly well in trendless markets that can't seem to make up their minds whether to go higher or fall further. Experienced price-momentum investors simply sit out such markets. This overview won't make you an expert on any of these strategies. And, oh yes, stick to it.