I'm not much of a good doubles played myself, but playing doubles definitely opens up the possibility for having more..."style", for need of a better word. In singles, there's almost always a best option available that you should take, or a worse option that may trip up the opponent if they try to make a read. But in doubles - and especially in PM doubles - there are a multitude of different strategies that could only work as a team effort.
Character-specific pairings. They're pretty cool, so go ahead and look some of them up, or try come up with some of your own. Not necessary, but helpful. Snake+MK/Kirby lets you get free kills by attaching a bomb to MK/Kirby and detonating it when they go off-screen with U-throw. Two Sheik/Zeldas on the same team allows for instant transformations, which helps a ton with recovery and detonating Dins. Lucario+a character that can survive a long time is good; the KB and damage of Lucario's entire moveset gets buffed, proportional to how few stocks he has compared to everyone else on screen. That means that he can go full on offensive while his partner stays back and holds stocks, and he'll just keep getting stronger with every death, until he can KO extremely quickly if you start falling too far behind.
There's also your team's shared mindset while fighting. There's a great thread somewhere...I think on the Melee boards...that discusses general improvement in Smash, and it covers a popular and efficient doubles strategy. Instead of thinking of doubles like it's a 2on2 battle, which can often separate into two 1on1s that change up ever so often, realize that...well, you're a team. Your partner should separate one of the opponents from the other and just haggle that guy down the entire match; nobody says you can't do 2on1, with the slower/less offensive opponent character getting knocked away whenever they try to help. Have one person on either side of the more offensive player/character (since the more defensive one will prob not be able to approach as easily), and make sure the person closest to the other opponent keeps hitting them away, almost like edge-guarding. Avoid killing the defensive player tho, since it's easier to knock them away for longer when they're higher %.
Of course, both of those are just examples I've read about or noticed from watching. There's prob a ton more strategies out there. But whether or not you choose to follow any of these, the more essential thing is to find one or two people you would be willing to team with often, and make them practice partners so that you learn each other's thought process. Develop strategies together. Practice against other friends in teams together. You're only half of the team, so if you wanna get better, it has to be a joint effort.