The vast majority of my matches are singles which I prefer, but I found a group of doubles players that I've enjoyed hitting with as well. However, I've never been coached and am not sure about my positioning and strategy (this for mostly 3.5, some 4.0 players). I usually just rely on being somewhat quick, general feel and assessing my opponents. But I'm sure being in better positions and learning more strategy will elevate my game.
Any tips or videos worth sharing?
One of the most helpful thoughts for doubles is the idea of keeping the ball low. In singles competition, we often try to keep the ball deep in the opponent's end. Lots of topspin and net clearance help with consistency while rallying from the baseline and a lot of the shot placement is about going cross-court or down the line.
But that extra net clearance that can be helpful in a singles baseline rally is a big liability in doubles when opponents at the net want poach across the middle and gobble up those rally balls that are up around shoulder or head height. Keep your shots down low where the other guys can't drive the ball, improve your positioning - this typically means taking over the net, force your opponents to hit up, and put the ball away when you see a reasonable opening.
Short-to-short and deep-to-deep. This means that when you're at the baseline, hit to the opponent at the far baseline (deep-to-deep) and not at the player at the net. If you're at the net, don't try to volley through an opponent at the far baseline. Either pop the ball toward an opening or zip it down into the feet of the opponent closest to you (short-to-short).
Our pals already pointed out the idea of positioning effectively with your opponent so that the two of you can be a sort of two-person wall. Try to be either together at the net or together at the baseline and not one-up-one-back. Also move together to the left and right so that you maintain a gap of about 8-10 feet between you. Communication with your partner is key for covering the court as a unit.
Singles players sometimes like to pound their first serves... a lot. I coach high school teams and those boys LOVE to wail on their first serves, but they also waste far too many of them and give their opponents a look at too many softer second serves. Fast serves in doubles can also be a liability when smart returners simply punch their returns right back at the server's feet and maybe leave that server stuck back on the baseline as they move forward.
Be aware of the effectiveness of your serves and consider your spin serve as a higher priority when playing doubles. If it takes a funky bounce like many spin serves do, that will force some weaker returns. But that slower flying serve will also give you time to get an extra step or two toward the net when you're following your serve forward. Your spin serves will probably also land at a much higher percentage than your heaters.
Lobbing... If you lob, send the ball deep - back near the far baseline. If you're playing outdoors, don't lob with the wind on your back - it will fly long too often. Much easier to lob effectively with a breeze in your face where you can send it deep and still keep it in. When your lob makes an opponent to turn and scramble after the ball, don't stand at your baseline and watch them run. MOVE IN and get ready to clean up!!!
Reaction volleys are a must for playing more effective doubles. Don't expect to get much better with hitting them by only playing some doubles. Get a lesson or recruit a hitting pal to help you out. With you hitting pal set up across the net from you at the "T" with a bucket of balls, have your pal pop balls at you at rather high frequency, but no more than moderate pace. Pop-pop-pop. When you get used to hitting a volley and immediately recovering to handle the next one over and over, you'll be a lot more comfortable and capable at the net when the heat turns up in your matches. A couple sessions of those reaction volleys can make a really big difference.