Groundstrokes should be hit crosscourt, especially at the beginning of the point.
In doubles, use the crosscourt (or diagonal) idea until you have your opponents in trouble and then change the direction or move forward. In singles, use the crosscourt to get the opponent moving and then use down the line groundstrokes when your opponent is off the court.
Crosscourt or diagonal shots pull your opponent away from the center of the court to a greater degree than down the line shots because of the angle of the shot. This opens up the court and should enable the player to hit a less risky shot to win the point.
Give yourself a lot of margin for error-the angle of these shots will enable you to hit the ball well inside the sideline and still pull the player wide, creating big openings on the next shot.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com). For details on lessons and camps at Sag Harbor Park Tennis and throughout the Hamptons this summer contact email@example.com or firstname.lastname@example.org