When you play a card, you should have a strong idea as to whether or not you'll win the trick with that card, and a strong idea as to whether or not you 'want' to win that trick. The card you play should match the strength of your desire to win (or lose) the trick. Really want to win it? Play the Ace. Really want to lose it? Play the 2.

So, as you progress through a hand of Hearts, keep track of where your middle cards are, relative to what's been played. A 9 can be a low card if the 2, 3, 4, 5, and 8 are out and you're sure that all 4 players have that suit; a 9 will also be high if the face cards and the 10 are out. Know which cards in your hand are likely to win, which are likely to lose, and play them that way, and you'll see the course of the game moving in the direction you want more often than not.