It's much easier to get the play right when you know which cards your opponents hold. For example, you already know that the normal play when you hold nine cards in a suit missing the Queen is to play the Ace and King rather than finesse. But if you knew that one of the defenders held three cards in the suit, you would always finesse against that defender.
The way to gather the information you need is to count the opponents' distribution and high cards. A significant portion of the time, you will be able to get a complete count of their holdings. Even when you can't get a complete count, a partial count will be enough for you to improve your chances.