sitting position is very important. if you are serious about being consistent with your double kicks, you should get a good drum throne and bring it on gigs.
work with a metronome, start slow, 16 beat at 130 bpm seems like a good speed to start on. do the exercise for 3 minutes straight l-r-l-r ... for about 30 mins or 10 sets with 1 minute rest in between.
the next day, increase it by 5 bpm, or until you cant be consistent, to which you will have to go back to the previous speed you were consistent on.
in no time, you will be fast, clean and efficient.
double kicks are like working out, since stamina and consistency are more important than actual musical skill.