Strategy - Return of Serve in Doubles
The number one goal on return of serve is to put the ball in play. Every time you hit the ball out of bounds or in the net, your opponents get a point! Those free points are very encouraging to a server! Conversely, making the serving team win every point is often discouraging to the server because they are used to getting some of those free points.
The number two goal on return of serve is to get to the net. It is easier to win points from the net than it is from anywhere else. The return of serve is the one time that you can usually beat the other team to the net because they have to let the ball bounce before hitting it.
The third goal on return of serve is to avoid hitting a short return. A short return allows the serving team a better chance of getting to the net easily with a dink or to hit a power shot at you which will be increasingly difficult to return the closer they are to you.
Returning the ball high and slow or hard and low
If you hit the ball hard and low on your return, you will probably get more errors or weak returns from the serving team, but you will usually make more errors yourself. You also have less time to get to net before the ball is returned at you.
If you hit the ball higher and slower, you will probably make less errors yourself, but will usually get less errors from the opponents. Keep in mind that a high deep return gives you a lot more time to run into net before the server gets to hit the ball. If you're tired or have bad knees or just have trouble moving into net for any reason, you should probably use a higher deep return when possible.
Every day and every game and every opponent is a different situation. One game you might be making all your hard low returns and they might be causing the other team to make errors, so you obviously wouldn't want to stop. On the other hand, you might suddenly start making faults yourself, in which case you should usually switch to a little higher, little softer return to avoid giving them points. If you and your partner both make two service return errors, that's already 4 points for the other team.
Remember the number one goal mentioned above, if you're having trouble doing what you would like with that tough serve and you're trying to win, just put the ball in play no matter what!
There are exceptions to every rule. You might one day find yourself playing a team that seems to win a point on their serve every time you return safely, but has trouble getting back your hard deep return. In that case, you might have to continue returning the serve hard and low even though you are making more errors than you like.