Mens doubles is relatively straightforward.
You can stand as far forward as possible, while still being able to deal with a flick serve. Obviously if you can kill a loose serve, you *always* should. Otherwise, you should play either a drop (straight or away from the server), a midcourt push to the tramlines, or a flat drive to the rear corners. Mix up your shot selection, but if you find one return is particularly effective then keep going back to it more often.
Don't lift unless you get badly caught out by a flick serve.

Mixed doubles is a little more complicated.
When the lady is serving, you can be more aggressive and stand further forward. At lower levels of play it's often possible to intimidate the lady into making a lot of mistakes (sorry to be sexist, but I think it's true). Bear in mind that if you're just playing socially, this isn't considered very gentlemanly conduct (don't expect to get her phone number after the game).
When the man (or a good lady) is serving, it's usually a good idea to stand further back and play slightly less aggressive returns. This means you won't always be able to kill slightly loose serves, but it also means you'll be in a better position to deal with the next shot. Otherwise if the opponents hit the shuttle behind you, you either have a long way to go to reach it, or your partner has to take it, which puts you in your weakest formation (with the lady at the back and the man at the front). Also, the man will be serving from further back in the court, so his serves will be flatter.

Shot selection also depends to some extent on who is serving. If the lady is serving, midcourt pushes to the tramlines are usually the most effective shot, as it brings the man forward and to one side (and also causes some confusion over who should take the shot). You can then follow that with a shot into the rear corner furthest from the man, which either he will take (in which case he may be late getting to it) or the lady will (in which case you have forced them into their weakest formation, with the lady at the back). For this reason, the midcourt push is also a good shot to play during rallies. As always, some variation in shot selection is still important.
If the man is serving, drives may be more effective than pushes. Assuming the server's partner is standing just left of the “T”, and he is also quite a long way forward, that leaves a lot of space behind them to hit into. Even if they're in level doubles formation (the lady not by the “T”), a shot to the rearcourt will still force / keep them in their weakest formation.

There are two key points to mixed. The first is disrupting the opponents' formation – try to get the lady at the back, as she won't have the power to do much damage from there. The second is getting on the attack (and avoiding lifting) – this is important in level doubles too, but even more so in mixed doubles, as the attacking power of the man causes a real mismatch for the opposing lady (once you're on the defensive, it's very hard to regain control of the point). This all starts with the return of serve.