Hrqls: If your opponent has a "free drop", you should double immediately. A free drop is present post-Crawford whenever the trailing player is an even number of points away from winning. For example, if the score is 1-away, 2-away, the leader loses nothing by dropping except the chance to win the current game. If the leader has less than a 50% chance of winning the current game, he should always drop. So, you should double before he has a chance to fall behind.

If you are an odd number of points from match, then the strategy can get more subtle. There's a certain amount of psychology involved. The leader now has something to lose (and you have something to gain) if he drops. In most cases he should take, but sometimes you can induce an opponent to drop by mistake if you wait. For example, you may have a position in which a good roll will give you a shot at gammon. You should double before rolling, since the opponent may drop and bring you one win closer to winning the match.