We have all heard the advice about hitting the ball to the middle in doubles whenever you are not sure where to go. This is very sound advice and usually works well.
Another idea is to always go towards the baseline player (if there is one) to set up the point or if you are in trouble. This will give you time to move forward or get back into the point if you are on the defensive. You can change the direction and go towards the net player once you feel there is a good chance of winning the point by hitting it in that direction.
Try to hit an extra shot to the baseline player whenever you are not sure about being able to win the point. Use the middle idea if you are in trouble or cannot be sure of your ability to control the direction of your shot.
It is also a great play to hit down the middle if both players are on the baseline or if both players are at net.