run or bike shuttle pickup chinese fast-feet drill shadow footwork drill if training alone on court. Not sure how experienced and what your age is, but 5 days a week for a month of 3 of the 4 above and you should already be seeing a significant improvement, provided your foundation in footwork for the most part is sound.
There's a recent thread about this already. http://www.badmintoncentral.com/forums/showthread.php/117576
chinese fast feet works wonders, shuttle pick-ups as well. running and cycling will help with the overall endurance, but won't necessarily help you with explosion on the court, foot work will. as you build on your endurance, you'll be able to do the footwork drills longer and longer. your core and back muscles are important too, work on those as much as you work on your feet and legs.
also, for running and cycling, what you can do is do intervals. if you're running, go 80% of your top speed for 2 minutes, then 60% of your top speed for 2 minutes. alternate it for about 25-30mi in the beginning. as you gain more endurance, prolong the sessions. cycling is the same thing with speed, however if you are riding up in areas that aren't always flat, the just go at regular uphill pace, then once you hit a particular flat area, do a quick sprint and ride as fast as you can maintaining a cadence of 90-100.
some good suggestions but that most obvious one missing, which is the best, is just play more singles if possible.
but it is impossible because the court is expensive so we play doubles most of the time. But I agree it kills. Just finished one amtch and it feels like having played doubles for the entire session without breaks