Deciding where to hit the ball in tennis is a tough call. So much effort and energy goes into perfecting your strokes, but to often we disregard this critical component of the game.
Just hit cross court! Don’t go down the line. Play it safe.
Paul Wardlaw built a super simple set of guidelines based on geometry that makes deciding where to play a given ball a very simple decision. Here is a link to a pdf of his article talking about the directionals: http://iamk.biz/directionals
It changed my game as a player and once you understand it, it can change yours too.