Playing from the baseline in doubles - court coverage 

If you decide to play from the baseline, in doubles, there are a few things to consider in order to be successful:
- keep the ball away from the opposing net player, most of the time. Why most of the time? Because sometimes you will have to hit the ball in his direction to keep him honest (make him stay put). 
- hit your ground-strokes deep cross-court if the opposing baseline player decides to stay back, or low cross-court if he comes up to the net.
- use topspin on your ground-strokes to make the ball come down early and low to the net player’s feet.
- don't use slice ground-strokes if you are behind the baseline; they float, giving the opposing net player time to poach/attack them.
- use the lob to keep the net players from charging the net. 

Covering the court from the baseline:
1. Serving
Consider serving from about 3-4 feet away from the baseline center. This tactic will allow you to place your serves closer to the T (service line center) and closer to the center of the court which sets-up your net partner for a poach. Don't worry about covering the cross-court return. You have plenty of time to cover the eventual cross-court return right after the serve delivery; you can be in position to cover it even before the returner is about to stroke the ball. 
Besides, if you look at graphic 1, you will see that if your serve is close to the center, the returner is obstructed by his partner to deliver the return at a sharp cross-court angle. So he'll be forced to place his return closer to the center of the court which gives your net partner a pretty nice look at a poach. 
Notice from graphic 2 that if you position yourself a little closer to the side line to serve, your delivery will be angled in the opposing court which opens up the possible angles of return: down-the-alley, sharp cross-court, lob, and deep cross-court. That wide serve delivery should be used sometimes only to add variety to your serve placement but you definitely should let your partner know your intention so that he will be more aware of the choices offered to the returner. 
If you choose to serve wide (close to the side line) then be prepared to cover a lot more court (compared to serving down-the-T) - move a little closer to the side line to cover the possible sharp cross-court return. 
So, in review, serving wide you will have to move wider to cover the cross-court compared to serving close to the center line where the return will come more to the center of the court. (see graphic 1)
2. Returning Serve
Position to return serve according to the server's position. If he is closer to the baseline center, then stay a couple of feet inside the singles side line. If the server stands wider (closer to the side line) then move over behind the singles side line to cover the possible wide angled serve. 
Your main delivery on the serve return should be away from the opposing net player. But this can be excluded if the speed of the serve is at a lower pace. In this case you can step into the court and attack the net player. 
If the first serve is powerful, then shorten the backswing and deliver the ball cross-court away from the opposing net player or block-and-lob it over the net player. 
Here is a trick you might want to use: 
- when the second serve is delivered to you, back up a little as the ball comes your way; that will give you a little more time to see what the opposing net player's intentions are: is he moving closer to the center to poach or does he stay put?
3. Rallying from the baseline 
Keep the ball cross-court away from the opposing net player with aggressive topspin ground-strokes.
Right after the delivery, position yourself so that you are in the middle of the possible angles from the opposing baseline player - the angle is formed by the opposing baseline player, your net partner and opposing net player. (see graphic 3)
If you decide to test the opposing net player, hit the ball aggressively in his direction. You will be responsible though for the next shot, just in case he is ready and makes good contact with the ball. He might respond either towards your net partner or the gap (between you and your partner); in either case, your partner will have very little time to react. Therefore it will be your job to cover the ball coming from the opposing net player. 
4. Lobbing
If you intend to lob the opposing net player and your shot is successful, then move up to the net in anticipation that the opposition will reply weakly or they will lob back. I would suggest that since you are the last player to get to the net (your partner being there already), you should position yourself a little further back than your partner. In this case you will be in a better position to cover a deep lob from your opponents. (see graphic 4)