Slow-playing is a technique intentionally used in order to make your opponent presume that he is in a much better situation than yourself. Slow-playing is generally efficient against both good and bad poker opponents (bad opponents simply bet their hand, so if they have something, they will pay you off no matter what).

It's not advisable to slow-play when there is a draw on the board; you want to slow-play when someone has a poker hand that is not very likely going to improve on yours. For example, suppose A95 is on the board. You have 99 and you are sure your opponent has an ace. It is better to wait until the turn to bet hard, so you can draw more money out of him for at least one extra big bet.