A simple strategy for doubles is what I call "under and over." Keeping the ball low to your opponents is paramount in doubles.
 
Normally, when you see your opponent on the baseline, common sense tells you to keep the ball in front of the baseline or "under" them. However, as you see your opponent moving forward toward you, a common mistake is to try to hit the ball to the baseline. I refer to this as "trying to beat the court."
 
A good way to stop this is to imagine the baseline is always moving in front of your opponent as he moves. This way, if you make a mistake of over hitting your shot it still has a chance of going in, as opposed to thinking you are good enough to hit the real baseline. The key in doubles is to force your opponents to hit "up," you do this by hitting "under" them.
 
There will be a time when your opponents are so close to the net that you would be smart to not try to hit "under" them. I call this position, when your opponent is too close to the net, your "personal space." When a player or team is in your "personal space", this is when you should go "over" them, usually with an offensive lob when you are more stationary (still) and a higher more defensive lob when you are forced to hit the ball on the run.
 
Many of my students will say to me "I really try to keep the ball low to the net." I refer to this as "trying to beat the net." In actuality, you are not trying to beat the lines on the court or the net. You are trying to beat your opponent. The pros do not see the baseline or the net; they choose their entire shot selection based on the position of their opponents. They are not trying to beat the court. They are trying to beat their opponent.
 
Try this "under/over" method next time you play doubles. It makes for a very simple strategy. Remember; let your opponent's position determine where you should hit the ball. Quit trying to hit balls low to the net or directly to the baseline or sideline. Force your opponents to hit shots they can't win with, as opposed to trying to make perfect shots, which are few and far between.
 
You will be better off using this "under/over" strategy in doubles, rather than the typical left/right strategy, which is more often used in singles. When you create an angle you have to be able to cover that angle. This is one of the most common mistakes in doubles.
 
Good luck on the courts!