Ok, I replied to this thread with a loooooong message and then decided to make an article out of it and cleaned it up. I am pasting over the original post with the article version I wrote, it is better with more insight. I hope it helps! it is painfully similar to the original response...
I have quickly learned that certain people I can trash in singles can smoke me in doubles. It took a decent amount of doubles before I really started to see the difference between the two. One particular item is the serve. Mixing up the serves is a good thing in doubles. But as you mix up your serve speed and placement please be aware of how it helps you and how it hurts you, as well as how it hurts and/or helps your partner at the net.
Going out wide with your serve can potentially put your doubles partner in somewhat of a vulnerable position if you use the serve too much. It is tough to poach the ball because the opponent has the alley to hit the ball into and the ball has the potential of coming back with some serious angle. Both of which when trying to cover as the net man can put your partner out of position. I think the best bet is to usually go down the "T" or right at their body which jambs them. If your opponent doesn't like being stretched out that far and hitting cross court shots than it can potentially be a plus for you and your partner. However, you would need to look out for more down the line returns.
By serving the ball down the middle of the court your opponent has less of an angle to return with and when they return the ball down the middle you have a better chance of covering the shot they hit. It also puts your partner in a much better position for a poaching situation.
Going right at their body forces them to get out of the way and sometimes even just block the ball back which allows your partner a lot more time to step in and take the easy shot. You would be surprised how many people underestimate the "in the body" serve. I know aces look better, but these types of serves are very effective. I got myself out of trouble with some "in the body serves" recently in the last game of a 3rd set and won the set. I actually aced him once going right at his body. The opponent returning serve was trying to get out of the way of the ball and became off balance and didn't have a chance to swing at the ball as it blew by him.
The other thing is you need to pay attention to are player's strike zones. I played a guy last night who was getting on top of my kick serves like it was no big deal and crushing it back, consistently... So I started hitting slice serves and he had a much harder time returning the low slice serves. Mixing up the serve and finding out what works against certain players is all a part of the game.
I play another gentleman regularly who is about 6'-4" tall and returns great and hits groundies great when he is at full stretch. My strategy against him is to jamb him which is more effective. He still gets some good returns back, but not as powerful for the most part. I also draw more errors out of him this way. The taller guys have more problems when they are jammed with a serve or a shot. Some of the shorter guys will not struggle with that as much because they have less body to get out of the way, but the out wide and high balls could give the shorter players some problems.
If you just bomb your serves in and your opponent lobs them back deep into the court every time it is time to rethink the big serve. Serves that have more spin and kick to them are harder to lob. Mixing in some slower pace serves with more spin could catch your opponent off guard. Or your opponent may have more confidence to return the serve with a groundstroke rather than a lob. If your opponent returns with a cross court groundstroke it will help you and your partner both be at the net as opposed to deep court lobs which push one player back to the baseline and allow your opponents to attack the net.
Another thing to keep in mind is what your partner expects you to do. Communicate with your partner what serve you will hit before you hit it. Although you do not see a lot of this in club play, it can be a huge factor in the game. I will let my partners know where I plan to serve so they can position themselves appropriately and not try to poach when I plan on throwing in an off pace slow ball serve.
In the end, adapt to who you have and who you have as a partner. Mix up the serves so your opponent doesn't get into a rhythm, but find what works and be sure to attack the vulnerable parts of your opponent. If you see them starting to lean in one direction or another on your serve, attack the open spot!