I have recently started playing Mixed Doubles with a close friend of mine. We have played one local high school tournament together just to see how well we could do and we did rather well considering the competition we were up against. Now we want to find ways to add more aggression into our play as we often end up in a defensive position.
You may be able to add aggression to your game by training without lifting (I think it's called the Danish warm-up or something), where you can only drop, smash, and drive. You could combine this with a 3v2 drill for added effect.
It's called Danish singles although the Danes themselves have a different name for the exercise!
Basically play a game on half court - the boundaries of the court are modified to the service line, the back inner doubles line, and the sides of the court to the centre line. The shuttle must land in this area - although the player can move anywhere. For example, the player can come right up to the front of the net.
As stated, the objective is to play pushes and drives keeping the shuttle low. It's quite hard to control the shuttle when starting out. Little tricks that help are crouching low, taking the shuttle in front of you, keeping the shaft more horizontally when you strike the shuttle.
Personally, I like counterattack turning defense into attack. All the great mixed pairs have a lady that can defend and turn the opponents smash into a counterattack just by defending with blocks or deft pushes off a smash.
Just do some smashes at your partner and see if she can block to the forecourt area. With a good block, the shuttle passes just above the net and is flat or going down. She then follows up by going to the net ready for the next shot. Don't smash fully as that would defeat the exercise if she cannot return the smash. Start with half smashes and then gradually increase the power in later sessions.
I used to watch videos for many hours to learn this. Park Joo Bong was awesome at it. Koreans now are very good at it. With today's doubles, if you can slow down and do analysis of a video, you find top pairs play a lot of flat drives, and crucially, half drives and pushes. If defending, watch for the return of smash that changes the rally around - what shot was played to cause it.
Ma Jin of China is very good at doing this as is Lilyana Natsir.
In addition to the above, practice transitioning with the lady at back to the front. You'd probably run into this quite often - your opponents would flick serve to the lady, and then pin her there. The flip side is also true - practice good low/flick serves, so the opponent man couldn't attack you easily, and you've a better chance to pin the (weaker) lady at the back.
The lady at the net should have net brush/kill shots, and willingness/guts/timing to execute them. This will deter opponents from playing setup net shots that force your side to lift.
Other aggressive situations - once you play a good shot (e.g. a mid-court push), have racket up immediately and look for the straight flat reply - hit down. This can be used in level-doubles as well. You can use it in serve/receive (the 4th shot).